Travel section of today's NY Times features Philadelphia. Also look for some travel reports, Amy did a brilliant one.

http://travel.nytimes.com/2013/01/06/travel/36-hours-in-philadelphia.html?ref=todayspaper&_r=0

> what are the most important sites that I should see?

Depends on what YOU consider important.
Want to see completely gross and disgusting things? Go to the Mutter Museum.
http://debakingpsychopath.blogspot.com/2010/11/mutter-museum.html
Want to see more Renoir paintings then any other place in the world? Visit the Barnes Collection.
Since EITHER of these -- and just about anything in between -- MIGHT be of interest to you, I'll not impose MY values on you by telling you where to find what *I* find fascinating. If you have specific interests, I'll be more than glad to direct you to where you can experience what YOU are interested in.

Because of little time that I have, I am interested in main historical sights of the city.

Probably about 2 days should do it, if all you want to see are the main historical sites.

Independence Hall: If memory serves me, you can do the tour in an hour or so. From there it's a short walk to the Liberty Bell, about another hour.

Also a short walk from IH: Betsy Ross House, also an hour or less. In the same general neighborhood, I suggest something a little offbeat: The Liberty Museum.

The Constitution Center: Budget about 3-4 hours to walk through and catch the live presentation.

Franklin Insitute: Half-day; check on what exhibits are available.

Of course, there are many other things to see and do in PHL, such as the Art Museum, but I'm giving you the barebones historical tour.

Start out at the Visitors' Center and get a timed (free) ticket to go through Independence Hall. Then the Liberty Bell.
You can walk to most of the Historic sites. Pick up a map at the Visitors' Center.
These sites lists acitivities, restaurants, sites, etc.:
http://www.uwishunu.com/
http://www.visitphilly.com/?gclid=CNW937q31rQCFQZnOgodk0gAbw

I'll just mention a few things I love most about Philly.

* Reading Terminal Market has a terrific ambience.
* Independence Hall tour by the rangers is well-done.
* The Art Museum is one of the best I've seen in the world.
* I love just strolling! In particular through Rittenhouse Square, South Street, the Italian Market and the colonial charm of the side streets between Independence Hall and Rittenhouse Square. However given that you're going in late January, it may be too cold for that.

> I am interested in main historical sights of the city

Independence Hall (as noted above)
Constitution Center
Franklin Court
Elfreth's Alley
Atwater Kent History Museum
Valley Forge (although this is a bit out of the way)

Liberty Bell Center and the Betsy Ross House are an interesting look at how places of no real historical value became places of symbolic value.

The colonial/Revolutionary history is concentrated in a few blocks, but if you have time, there are many more areas of the city that are easily accessed by public transportation and equally worthwhile.

The historical "musts" are Independence Hall (including Congress Hall) and the Liberty Bell (which is really more about the abolitionist movement than the Revolution.) If you start at the Visitors' Center at 6th and Market, you can get oriented and get a map of the various sites. A nice walk (even in January weather) takes you from the center past the new President's House site to the Liberty Bell and on to Independence Hall, then a continuation to Carpenters' Hall via the Dolly Todd (Madison)house and a view of the portraits and statues at the Second National Bank takes in quite a bit of early history. For the lower end of "Old City", there's a walking tour here; Franklin Court is mostly closed now, though:
http://www.fodors.com/community/united-states/a-franklin-walking-tour-of-philadelphia.cfm

If you have more time, the Constitution Center is interesting (once, IMHO) and there are some other museums in the area: the newly renovated Philadelphia History Museum (formerly Atwater Kent) and the National Museum of American Jewish History, for two. Franklin Court's museum is closed for renovations, but the skeleton of his house is still there==and the Chemical Heritage Museum is right beside it, and both free and interesting for a short visit.

Further afield there is the Wanamaker department store (Macys) with its organ recitals and great architecture, with City Hall right across (tours available; City Hall is a bit over the top, but I love it.) Broad Street has the Furness Pennsylvania Academy of the Fine Arts as well as the Masonic Temple, both interesting architecturally, and you're also in the area of the Reading Terminal Market there, with its variety of food vendors and farmers' market.

One of my favorite places in the city is the University of Pennsylvania Museum of Archaelogy and Anthropology, but that's a bit of a ways away.

Of course the art museums mentioned above, including the Rodin, are a very worthwhile use of your time. Here's hoping the weather is good for you, and happy traveling!

www.visitphilly.com is a great site for planning.