There will not be a problem if your visa expires while you are in the United States. When you entered the U.S., the Customs and Border Protection Immigration Officer at the port of entry authorized you to stay in the U.S. for a set period of time when they admitted you into the country. They should have noted that period of stay on your Form I-94, Arrival/Departure Record. You can stay for the entire period noted on the I-94, even if your visa expires during that period of time. It is important, however, to keep a copy of your I-94 so that you have an official record of your permission to be in the United States, particularly if your visa does not reflect your entire authorized period of stay.
