A method, a user equipment, and a base station for maintaining uplink synchronization in a wireless communication system supporting carrier aggregation are discussed. The method of maintaining uplink synchronization at a user equipment according to an embodiment includes receiving a physical downlink control channel (PDCCH) order. The PDCCH order indicates an initiation of a random access procedure and includes a carrier indicator field and a predefined set of values of a 1-bit flag indicating a localized/distributed resource block assignment, and a resource block assignment field. The method further includes transmitting a random access preamble through an uplink component carrier corresponding to a value of the carrier indicator field among a plurality of uplink component carriers.