I am a nonresident aliens ans buying a house. I know a nonresident aliens can't be qualified for first home buyer tax credit. However, my residency status started to be changed to resident aliens for tax report from Jan. 1st this year and it will be changed to resident aliens from Jul. 3dr this year based on substantial presence test. My question is that whether I am eligible to get the credit as soon as closing of the buying a house or after Jul. 3rd. Or can I apply the tax return late of this year or at next year tax report?

When did you enter the U.S. and on which visa? Your current visa status?
If you are filing resident tax return for 2010, then you can claim homebuyer credit on 2010 tax return if you purchase your home.

Thank you, MukatA.

I entered the U.S. on May 15th 2008 and my VISA status has been J1. Do you mean I can claim it on only 2010 tax return, not as soon as purchasing the house?

Assuming you are not a student, you are exempt from residency for 2008 and 2009 so you must file nonresident tax returns.
In 2010, you can file resident tax return.