The present invention relates to a wireless communication system. A method for transmitting CSI by a terminal in a wireless communication system according to an embodiment of the present invention comprises: subsampling a first codebook associated with a first PMI and a second codebook associated with a second PMI according to a report sub-mode for four antenna ports; and reporting channel state information based on the subsampled first codebook and the subsampled second codebook, wherein a first codebook index for the first PMI may be determined as one of 0, 4, 8, and 12 when RI is 1 or 2, a second codebook index for the second PMI may be determined as one of 0, 2, 8, and 10 when the RI is 1, and the second codebook index for the second PMI may be determined as one of 0, 1, 4, and 5 when the RI is 2.