I will be going to Paris in September and would like to take a day trip to Amsterdam to visit Anne Frank's house. Is it a long trip. Are there train stations that will take me directly there?

It's not an easy day trip -- it's 4 hours each way on the high-speed Thalys train.

Paris Gare du Nord to Amsterdam Centraal Station. Too far for a day-trip. You could also fly (1 hour flight), but too expensive probably.

Don't know but Amsterdam is a must

I agree with the previous posters who point out that the Thalys is one of the best ways to go and that it is too much for a day trip. Moreover, the Anne Frank House is interesting, but far from worth that kind of expense and trouble to see on its own. I would regard it as an incidental thing to visit in Amersterdam. You would really be shortchanging a great place to make that your mission in such a short period of time, IMHO.