A method is provided for a user equipment (UE) operating in a wireless communication system. The UE is configured with a first medium access control (MAC) entity and a second MAC entity. The UE triggers a power headroom reporting (PHR) in the first MAC entity if a condition is satisfied. A PHR MAC control element (CE) is generated for the triggered PHR. The UE transmits the PHR MAC CE to a first base station. The condition includes: there is an uplink (UL) transmission on any cell of the second MAC entity, and a required power backoff for the any cell of the second MAC entity has changed more than a threshold.