A method for reporting uplink control information and a wireless communication device using the same are disclosed. The wireless communication device supports multiple component carriers (CC), and the proposed method includes following steps. When a base station requests the wireless communication device to transmit aperiodic channel state information (CSI) report of one or more downlink CC to the base station, but the CSI of the downlink CCs may be invalid, following steps are executed on the downlink CC. Full payloads of channel quality indicator (CQI)/precoding matrix indicator (PMI) corresponding to a plurality of selectable RI values of the downlink CC are respectively calculated. Additionally, an RI value of the downlink CC is selected according to the full payloads of the CQI/PMI corresponding to the selectable RI values.