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: configuring multiple radio bearer groups (RBGs), each of the multiple RBGs including one or more radio bearers; receiving an uplink grant; selecting a RBG according to a predetermined rule; generating a Medium Access Control Protocol Data Unit (MAC PDU) containing a data of the selected RBG; and transmitting the MAC PDU using the uplink grant, wherein the predetermined rule is at least one of: selecting a RBG having a logical channel with the highest logical channel priority, selecting a RBG having a largest amount of data stored in a PDCP or RLC buffer, or selecting a RBG having a radio bearer with a shortest delay requirement.