A method and apparatus for transmitting control information of a small block size with high reliability in a mobile communication system supporting uplink packet data service are provided. A 6-bit Absolute Grant indicating an allowed maximum data rate for uplink packet data transmission is generated and a 16-bit User Equipment Identifier Cyclic Redundancy Check is generated by combining a Cyclic Redundancy Check with a User Equipment Identifier. The User Equipment Identifier specific Cyclic Redundancy Check and 8 tail bits are added to the 6-bit Absolute Grant and the added bits are encoded at a coding rate of 1/3. The resulting 90 coded bits are rate-matched according to a predetermined rate matching pattern and transmitted to a User Equipment. The rate matching pattern is {1, 2, 5, 6,7, 11, 12, 14, 15, 17, 23, 24, 31, 37, 44, 47, 61, 63, 64, 71, 72, 75, 77, 80, 83, 84, 85, 87, 88, 90}.