The present invention relates to a wireless communication system. More specifically, the present invention relates to a method and a device for transmitting data in a wireless communication system. According to an aspect of the present invention, the method comprising: receiving, from an enhanced Node B (eNB), a grant with an indication that indicates at least one logical channel group (LCG) of which data should be transmitted using the received grant, wherein each of radio bearers (RBs) configured to the UE is associated with a respective logical channel priority; generating a medium access control protocol data unit (MAC PDU) including data of RBs belonging to the indicated at least one LCG; and transmitting the MAC PDU using the grant.