A method for PDCCH allocation in a network terminal of a wireless communication system is disclosed. The wireless communication system supports Carrier Aggregation, which enables a UE of the wireless communication system to perform transmission and/or reception through multiple carriers. The method includes steps of configuring a plurality of carriers to a UE via an RRC message, and including an information for each configured carrier in the RRC message to indicate a second carrier which carries the Physical Downlink Control Channel (PDCCH) corresponding to a first carrier, wherein the PDCCH on the second carrier is used to transmit downlink assignment or uplink grant for the first carrier.