A user equipment (UE) and a base station (BS) for a mobile communication system are provided. The UE performs a random access (RA) procedure with the BS in a radio resource control (RRC) idle mode. The UE transmits a preconfigured uplink resource request message to the BS and receives a configuration message indicating a preconfigured uplink resource from the BS. The UE starts a time alignment timer (TAT) which is used for the RRC idle mode in response to the RA procedure. When the UE determines that there exists an uplink message to be transmitted in the RRC idle mode and determines that the current timing advance (TA) value is valid, the UE transmits the uplink message on the preconfigured uplink resource.