Provided is a method of processing hybrid automatic repeat request (HARQ) and adaptive transmission of a device-to-device (D2D) link. In the method, an operation method of user equipment (UE) includes (a) receiving an initial transmission mode (TM) and transmission power for the D2D link from a base station, and performing data transmission through the D2D link, (b) determining, at the UE, a TM and transmission power of a succeeding subframe, or receiving a TM control value and a transmission power control value from counterpart UE of the D2D link and determining the TM and the transmission power of the succeeding subframe, and (c) performing the data transmission to the counterpart UE using the determined TM and transmission power. Here, (b) and (c) are repeated.