A method of transmitting a data block in a wireless communication system includes generating a data block having a variable size in an upper layer, transmitting a radio resource request message to a base station according to the size of the data block in a medium access control (MAC) layer and transmitting the data block by using a radio resource allocated by using the radio resource request message. For a packet service such as VoIP in which a delay time is important, QoS can be improved by decreasing the delay time of packet transmission and by decreasing a packet discard ratio.