The present invention relates to a method and an apparatus for quantizing channel state information (CSI). An embodiment of the present invention provides a method for providing codeword index information to a base station configured with Nt directional antennas, including: determining a first codeword in a first codebook whose rank is r, where r=1, 2, . . . , Nt, and sending an index of the codeword to the base station; and determining a second codeword in a second codebook whose rank is r, and sending an index of the codeword to the base station. The first codeword is a block diagonal matrix, and a matrix on a diagonal block of the block diagonal matrix is a matrix with Nt/2 rows and Nb columns. The Nb columns are Nb different Discrete Fourier Transform (DFT) vectors arbitrarily selected from N predefined DFT vectors, N is an integer power of 2, 1≦Nb≦N, and the length of each DFT vector is Nt. Each column corresponding to each data stream in the second codeword includes an equal-length column selection vector for individually selecting from the Nb columns in the first codeword the DFT vectors used in two polarization directions and co-phasing information θ=2πm/M between the two polarization directions, where M is an integer power of 2, and 0≦m≦M−1.