A method of performing device-to-device (D2D) communication by a user equipment (UE) in a wireless communication system includes receiving information on a synchronization type for the D2D communication; based on the information on the synchronization type, selecting a synchronization source for the D2D communication; and transmitting information on the synchronization source used by the UE. Further, based on the synchronization type being a first synchronization type, the UE uses a cell of a network as the synchronization source and based on the synchronization type being a second synchronization type, the UE uses an external synchronization source as the synchronization source.