A method and apparatus for performing a device-to-device (D2D) operation in a wireless communication system is provided. In one embodiment, a user equipment (UE) receives a configuration of a reference timing for a cell of first carrier, based on a timing of a primary cell (PCell) of a second carrier, and performs D2D operation with another UE on the cell of the first carrier according to the reference timing. In another embodiment, a UE allocates a D2D transmission power based on an uplink (UL) transmission power for dual connectivity, and performs D2D operation with another UE according to the D2D transmission power.