A method and apparatus for transmitting effective channel status information (CSI) in a wireless communication system supporting multiple carriers are disclosed. The method for indicating CSI transmission in a wireless communication system supporting multiple carriers includes transmitting downlink control information (DCI) including a channel quality information (CQI) request field to a user equipment (UE) over a physical downlink control channel (PDCCH), and receiving CSI reporting of a single downlink carrier or multiple downlink carriers from the UE over a physical uplink shared channel (PUSCH) without receiving uplink data. If the CSI for the single downlink carrier is reported, the number (NPRB) of resource blocks (RBs) established for the CSI reporting is identical to or less than X (NPRB≦X), and if the CSI for the multiple downlink carriers is reported, NPRB is identical to or less than M (where M>X).