A method for a user equipment (UE) operating in a wireless communication system including a first base station (BS) and a second BS, includes configuring a first medium access control (MAC) entity to map a first logical channel set transmitting first data to the first BS and a second MAC entity to map a second logical channel set transmitting second data to the second BS, wherein the first logical channel set includes one or more logical channels and the second logical channel set includes one or more logical channels, if the first data belongs to a logical channel having a higher priority than priorities of logical channels for which data is already available for transmission in the first logical channel set, triggering, by the first MAC entity, a first buffer status report to the first BS regardless of priorities of logical channels of the second logical channel set.