The present invention relates to a wireless communication system. More specifically, the present invention relates to a method and a device for transmitting a power headroom reporting for LTE-WLAN aggregation system, the method comprising: configuring Long Term Evolution (LTE) network and Wireless-LAN (WLAN) network over which the UE transfers data in LTE-WLAN aggregation system; receiving a cell index used for the WLAN network from an eNB, wherein the cell index for WLAN network is currently not used for any cell of the LTE network configured for the UE; and transmitting a Medium Access Control (MAC) Control Element (CE) including information for the WLAN network with the cell index, wherein the cell index indicates that the MAC CE includes the information for the WLAN network.