A method of performing uplink synchronization in a wireless communication system includes transmitting a random access preamble which is randomly selected from a set of random access preambles, receiving a random access response, the random access response comprising a random access preamble identifier corresponding to the random access preamble and a time alignment value for uplink synchronization, starting a time alignment timer after applying the time alignment value, starting a contention resolution timer after receiving the random access response, wherein contention resolution is not successful when the contention resolution timer is expired, and stopping the time alignment timer when the contention resolution timer is expired.