1. A downlink transmission method by a base station in a mobile communication system, the method comprising:identifying a set of resource blocks associated with a control channel for a terminal;
transmitting, to the terminal, information on the set of resource blocks on a radio resource control signal, the information including first information associated with a number of resource blocks of the set and second information associated with an index corresponding to the resource blocks of the set;
transmitting, to the terminal, control information including information for downlink data on the control channel identified based on the first information and the second information; and
transmitting, to the terminal, the downlink data on a data channel based on the control information,
wherein the set of resource blocks includes at least one control channel resource, and
wherein a search space for the control channel of the terminal is defined based on an aggregation level, a number of the at least one control channel resource, and a number of a candidate associated with the control channel.