A method and a user equipment (UE) are described for transmitting power headroom (PH) information in a wireless communication system. A primary cell and at least one secondary cell are configured with the UE. One logical channel identifier (LCID) field is configured in a medium access control (MAC) sub-header. The one LCID field indicates a presence of a PH MAC control element (CE). A MAC protocol data unit (PDU) comprising the MAC sub-header and the PH MAC CE is transmitted. The PH MAC CE includes PH information for both the primary cell and the at least one secondary cell.