A method, a user equipment (UE) and a communication system are provided. In the disclosure, a UE configured with multiple carriers determines whether a first, a second, and a third conditions are met, and then determines whether resources allocated for the UE on the carrier are sufficient or not accordingly. The first condition defines that the UE is transmitting as much scheduled data as allowed by a current service grant on the carrier. The second condition indicates that the UE has enough transmission power available to transmit data at higher data rate on the carrier. The third condition indicates total buffer data requires more than a preconfigured time period to be transmitted through the carrier together with at least one of other carriers among the multiple carriers.