The present invention relates to a wireless communication system. More specifically, the present invention relates to a method and a device for performing a LCP procedure in a carrier aggregation, the method comprising: configuring with a first cell group including at least one first cell and a second cell group including a least one second cell served by an eNB; receiving two priorities for a logical channel for a LCP procedure; receiving a UL grant from the first cell or the second cell; selecting a priority based on whether the UL grant is received from the first cell or the second cell; and performing a LCP procedure for the logical channel using the selected priority.