The present invention provides a method for performing Buffer Status Reporting (BSR) in a user equipment (UE) of a wireless communication system. The method includes steps of forming a Medium Access Control (MAC) Packet Data Unit (PDU), the MAC PDU including padding bits with a bit amount larger than the size of a BSR MAC control element, triggering a padding BSR procedure, reporting a short BSR MAC control element when the padding bits of the MAC PDU cannot accommodate a long BSR MAC control element, and indicating the short BSR MAC control element triggered by the padding BSR procedure through a sub-header corresponding to the BSR MAC control element in the MAC PDU.