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 a first uplink grant for a logical channel group (LCG) and a first LCG indicator of the LCG; generating a Medium Access Control Protocol Data Unit (MAC PDU) containing a data of only the LCG indicated by the first LCG indicator; and transmitting the MAC PDU using the first uplink grant.