The San Onofre Nuclear Generating Station was shut down due to safety concerns arising after unusual early wear was detected in one of the plant's reactors.
The NRC was reviewing a request by Southern California Edison to restart the plant after repairs were reportedly made. A decision by the agency was originally expected as soon as late April. The NRC website said Tuesday the decision will be delayed until May or June.
San Onofre's reactors have not produced electricity since January 2012, when a small radiation leak led to the discovery of unusual damage to hundreds of tubes that carry radioactive water.
NRC spokesman Victor Dricks said agency staff needed more time to review technical data submitted by Edison.
The Associated Press contributed to this report.