Mr Obama said he had no choice but to act, accusing the Republicans of blocking a bill in Congress, and vowed to make permanent reform a reality.
The current move means that more than four million illegal immigrants will be allowed to apply for work permits.
Republicans say it will encourage more people to arrive unlawfully, informs Report citing BBC News.
Republicans have been weighing their response to Mr Obama's executive action, but the dilemma they face is how to oppose his immigration reforms without alienating Hispanic voters - whose support they will need in the next election, the BBC's David Willis in Washington reports.
Speaking in Las Vegas, where he first outlined his plans two years ago, President Obama said: "I will never give up."
He said: "We're going to keep on working with members of Congress to make permanent reform a reality.
"But until that day comes, there are actions that I have the legal authority to take that will help make our immigration system more fair and more just, and this morning I began to take some of those actions.
"When members of Congress question my authority to make our immigration system work better, I have a simple answer - pass a bill."