I would like to know if a expatriate Brazilian citizen working in the United States have to pay tax in the US after to sold a property in Brazil. Thank you very much.

If you are resident of U.S. you must report worldwide income on your tax return. You are resident of U.S. if you meet Substantial Presence Test. Your U.S. Tax Return: Substantial Presence Test (http://taxipay.blogspot.com/2008/04/substantial-presence-test.html)

Thank you very much. I work in an American company in Brazil and I'm being transferred to the U.S. headquarters from 01/01/2011.

I plan to sell a property in Brazil and I will pay taxes in Brazil. I believe it is possible to avoid paying new taxes in the United States, correct ?

Yes, IF you sell the property BEFORE you achieve residency status in the U.S.