A method and apparatus for initiating communications on a shared channel in a mobile communication system are provided. A user equipment transmits an uplink signal for requesting communications on the shared channel to a Node B, and waits for a delay duration without monitoring a downlink after transmitting the uplink signal. When the delay duration has elapsed, the user equipment monitors the downlink during a valid period and determines whether a downlink signal responding to the uplink signal is received in a transmission time interval within the valid period. When the downlink signal is received in the valid period, the user equipment transmits and receives data on the shared channel.