The present invention relates to a wireless communication system. More specially, the present invention relates to a method of maintaining uplink synchronization at a user equipment in a wireless communication system supporting carrier aggregation; configuring plural uplink component carriers, wherein each of the plural uplink component carriers has a respective uplink synchronization timing; receiving a downlink control information (DCI) through a physical downlink control channel of a specific downlink component carrier, wherein the DCI indicates an initiation of random access procedure and includes a carrier indicator field indicating one of the plurality of uplink component carriers; and transmitting a random access preamble through a uplink component carrier indicated by the carrier indicator field among the plural uplink component carriers.