The present invention relates to a wireless communication system. More specifically, the present invention relates to a method and a device for transmitting, by a user equipment (UE), a buffer status report (BSR) in a wireless communication system, the method comprising: triggering, by a medium access control (MAC) entity, a buffer status report (BSR); and transmitting the buffer status report, wherein the buffer status report includes a packet data convergence protocol (PDCP) buffer size for a radio bearer only if PDCP BSR condition is met.