My father entered U.S.
illegally, and I entered U.S. as F1 student before. As a U.S. citizen
now, can I sponsor his immigration application?
If the your father entered
the U.S. without inspection - illegally without passport or visa, as a
general rule, he will have to return to the his native country and have
the interview at the U.S. Embassy or Consulate there, after you file
Form I-130 for him.
The problem with having a consular case for persons who have remained
in the US in unlawful status for more than 180 days, and they leave
before the U.S. permanent residence case is approved, then they will be
banned to return to the U.S. for 3 years. If the person remained
unlawfully in the US for a whole year or more, then he will be banned
of returning to the U.S. for 10 years. If he can avoid leaving the U.S.
in the first place, he cannot be banned from returning.