A method and apparatus for transmitting effective channel status information (CSI) in a wireless communication system supporting multiple carriers are disclosed. A method for transmitting CSI of DL transmission via uplink in a wireless communication system includes transmitting a rank indicator (RI) at a first subframe, and transmitting a first precoding matrix indicator (PMI), a second PMI, and a wideband (WB) channel quality indicator (CQI) at a second subframe. UE preferred precoding matrix is indicated by a combination of the first PMI and the second PMI. A subsampled codebook for each precoding codebook of ranks from Rank-1 to Rank-4 is applied to the first PMI and the second PMI, and a sum of the first PMI and the second PMI for each of Rank-1 to Rank-4 is comprised of 4 bits.