UE-specific search spaces (UE-SS) for a carrier-aggregated communication system are introduced to decrease the number of blind decoding attempts, decrease downlink control information (DCI) blocking probability, and maintain good blind decoding performance. In the proposed control channel allocation method, the control channel searching method and the communication apparatus thereof, the UE-specific search spaces are extended except control channel element (CCE) aggregation level one. Further, a new CCE aggregation level is created in the UE-specific search spaces. The sum of the number of control channel candidates for all aggregation levels is bounded by the maximum number of PDCCH candidates. In addition, uplink MIMO grant command is just allocated in a pre-configured component carrier or a set of pre-configured component carriers, but uplink MIMO grant command is not allocated in the remaining component carriers.