A method for reporting channel state information (CSI) by a user equipment (UE) in a wireless communication system, the method includes jointly encoding a rank indicator (RI) and a first precoding matrix indicator (PMI) into an encoded value for a specific CSI report mode; and reporting the encoded value, wherein the RI and the first PMI are jointly encoded into the encoded value in a way that following conditions are satisfied: when the RI is 1 and a codebook index for the first PMI is same as the encoded value, the encoded value is an integer ranging from 0 to 7, when the RI is 2 and the codebook index for the first PMI is the encoded value minus 8, the encoded value is integer ranging from 8 to 15, when the RI is 3 and a first precoding matrix corresponding to the first PMI is an identity matrix.