1. The Green Card Application for U.S. Citizen's Parents

A legal immigrant is a foreign national who has been granted the privilege of living and working permanently in the United States. There is a two-step process for your parent to become a legal immigrant. 

First, the USCIS must approve an immigrant visa petition that you file for your parent. Second, if your parent is outside the United States, your parent will be notified to go to the local U.S. consulate to complete the processing for an immigrant visa. If your parent is legally inside the U.S., he or she may apply to adjust his or her status to that of a lawful permanent resident using the Form I-485.

To qualify as a parent of a U.S. citizen, a person must meet the similar test as for the child, it means that if the U.S. citizen is adopted, the adoption must have been finalized before the child's 16th birthday, the parent must have legal custody of the child for two years (before or after the adoption), and the child must reside with the adoptive parent for two years (before or after the adoption); and if the U.S. citizen is a stepchild of the alien parent, the current marriage of the patent must have been taken place before the child's 18th birthday.  

2. The Two Scenarios for Parents of U.S. Citizens to Apply for Immigration

There are two scenarios for parents of U.S. citizens to apply for immigration:

1) The alien parent is already in the United States: the first scenario is that the alien parent is already in the United States in a nonimmigrant status or parole. In this case, the U.S. citizen may file an immigration petition to U.S. Citizenship and Immigration Services (USCIS) for the parent, and the parent can file an application to adjust status to permanent resident at the same time. The parent has not to wait for the immigrant visa number to become current before he or she may apply to adjust to permanent resident. 

2) The parent is outside the United States: In this case, the U.S. citizen needs to file an immigration petition and request that the U.S. Citizenship and Immigration Services to notify a U.S. Consulate in the country where the parent lives. Once the immigration petition is approved, the National Visa Center of the U.S. State Department sends a forms and information package to the U.S. citizen's parent. After the necessary forms are completed, the parent goes to the U.S. Consulate overseas to apply for an immigrant visa. On the day that the parent enters the United States on an immigrant visa, he or she becomes a U.S. permanent resident.