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), data in a wireless communication system, the method comprising: receiving an uplink grant including first transmission time interval (TTI) duration information; transmitting a Medium Access Control Protocol Data Unit (MAC PDU) using the uplink grant, wherein the MAC PDU includes data associated with at least one radio bearer supporting a TTI duration indicated by the first TTI duration information.