A method of handling uplink synchronization for a mobile device capable of receiving and transmitting on a plurality of component carriers in a wireless communication system, the method comprising: performing a first random access procedure on a first component carrier of the plurality of component carriers to establish uplink synchronization on the first component carrier; and performing a second random access procedure on at least a component carrier of the plurality of component carriers, which does not include the first component carrier, to establish uplink synchronization on the at least a component carrier, which comprises: receiving at least a random access preamble assigned by a network of the wireless communication system for the at least a component carrier; sending at least a random access preamble message including the at least a random access preamble to the network; and receiving a random access response message on the first component carrier corresponding to the at least a random access preamble message from the network, wherein the random access response message includes at least a timing advance value corresponding to the at least a component carrier and does not include a timing advance value corresponding to the first component carrier, determined by the network according to the at least a random access preamble message. performing a first random access procedure on a first component carrier of the plurality of component carriers to establish uplink synchronization on the first component carrier; and performing a second random access procedure on at least a component carrier of the plurality of component carriers, which does not include the first component carrier, to establish uplink synchronization on the at least a component carrier, which comprises: receiving at least a random access preamble assigned by a network of the wireless communication system for the at least a component carrier; sending at least a random access preamble message including the at least a random access preamble to the network; and receiving a random access response message on the first component carrier corresponding to the at least a random access preamble message from the network, wherein the random access response message includes at least a timing advance value corresponding to the at least a component carrier and does not include a timing advance value corresponding to the first component carrier, determined by the network according to the at least a random access preamble message. receiving at least a random access preamble assigned by a network of the wireless communication system for the at least a component carrier; sending at least a random access preamble message including the at least a random access preamble to the network; and receiving a random access response message on the first component carrier corresponding to the at least a random access preamble message from the network, wherein the random access response message includes at least a timing advance value corresponding to the at least a component carrier and does not include a timing advance value corresponding to the first component carrier, determined by the network according to the at least a random access preamble message. <EOS>