A method and an apparatus for efficiently transmitting or reporting a Power Headroom Report (PHR) of a User Equipment (UE) are provided. The method of transmitting the PHR of the UE in a mobile communication system includes configuring an extended PHR including an indicator corresponding to a variation factor of a maximum transmission power of the UE, and transmitting the extended PHR from the UE to a Base Station (BS). The BS may be notified of a maximum transmission power of the UE and a variation factor corresponding to the maximum transmission power in order to enable efficient scheduling.