A method of reporting a channel state to a base station supporting downlink multiple input multiple output (MIMO) in a wireless communication system includes measuring a downlink channel based on a downlink signal received from the base station, and reporting a codebook index for precoding of the downlink signal according to the measurement of the downlink channel. In the case in which the UE receives channel state information (CSI)-reference signal (RS) resources for N1 antenna ports, if a report for N2 (N1>N2) antenna ports is set, a codebook for the N2 antenna ports is configured as a subset of a codebook for the N1 antenna ports configured by the base station. Information about the N2 antenna ports is received from the base station.