A method and an apparatus for receiving downlink data in a multi-carrier system are provided. A terminal receives a downlink grant on a physical downlink control channel (PDCCH) via a first component carrier, and determines a transmission power of a physical downlink shared channel (PDSCH) depending on whether a control channel exists in each orthogonal frequency division multiplexing (OFDM) symbol of a subframe of a second component carrier that is indicated by the downlink grant. Based on the transmission power of the PDSCH, the terminal receives downlink data on the PDSCH indicated by the downlink grant via the second component carrier. When a control channel and a data channel coexist in the same OFDM symbol, the reliability of receiving the control channel can be increased.