A method and apparatus for controlling transmit power of a User Equipment (UE) in a mobile communication system is provided. The transmit power control method of the present invention includes determining, when the terminal's required transmit power exceeds the terminal's maximum transmit power, whether a plurality of Hybrid Automatic Repeat reQuest (HARQ) processes are running and reducing, when the plurality of HARQ processes are running, a transmit power of at least one of the plurality of HARQ processes which has not experienced transmit power reduction previously. The UE transmit power control method and apparatus of the present invention is advantageous to control the transmit power in transmit power reduction mode.