The present invention relates to a wireless communication system. More specifically, the present invention relates to a method and a device for processing an uplink grant after the last uplink transmission in wireless communication system, the method comprising: transmitting uplink data with a buffer status reporting (BSR); receiving uplink grant after the BSR is transmitted; and discarding the received uplink grant if the received uplink grant is for new transmission and a buffer size of the BSR is set to zero.