A method of transmitting control information in a carrier aggregation system is provided. The method includes: transmitting bandwidth information of a component carrier; configuring downlink control information including an additional indication field and an information field; mapping the downlink control information in any one of containing units having a plurality of predetermined different bit sizes in accordance with the bandwidth information; and transmitting the downlink control information mapped to any one of the containing units, wherein the information field is any one of a plurality of control information formats applicable to the component carrier, and the additional indication field includes information indicating a specific control information format to which the information field corresponds among the plurality of control information formats. Accordingly, the number of blind decoding attempts of a user equipment can decrease in a carrier aggregation system. Therefore, power consumption of the user equipment can decrease, and a decoding rate can increase.