A method, adapted to a communication device, for uplink transmission by using unlicensed spectrum is provided. The method includes following steps. Acquiring transmission resource based on a listen-before-talk (LBT) procedure. Sending an LBT indicator to a base station after successfully acquiring the transmission resource. Receiving a uplink resource grant from the base station. Performing uplink data transmission with the base station on the unlicensed spectrum according to the uplink resource grant.