A method for soft-combining MBMS (Multimedia Broadcast/Multicast Service) data in an asynchronous mobile communication system including a user equipment (UE) and a radio network controller (RNC), wherein the system provides an MBMS service. In the method, the RNC measures a round trip delay (RTD) for a primary cell in which the UE is located, and an RTD for each of the neighbor cells neighboring the primary cell. The RNC transmits the MBMS data at the same transmission time by considering the RTDs for the primary cell and the neighbor cells so that the UE can receive requested MBMS data. The UE receives MBMS data transmitted from the respective cells at the same transmission time, and performs soft combining on the received MBMS data.