A method of relay transmission in a mobile communication system. The mobile communication system includes at least one base station and a plurality of user equipment units (UEs). When one of the UEs attempts to send a message to the base station, this one of the UEs, referred to as a requesting UE, is capable of using the method of relay transmission to send messages to the base station via another one or more UEs, referred to as relay UEs. The method of relay transmission includes the following steps. First, a relay request confirmation procedure for the requesting UE and the relay UE is performed. Second, a base station confirmation procedure for the requesting UE, the relay UE, and the base station is executed. Next, an identity authentication procedure for the requesting UE, the relay UE, and the base station is performed. After that, a relay start confirmation procedure for the requesting UE and the relay UE is executed. Finally, a relay transmission procedure for the requesting UE, the relay UE, and the base station is performed, wherein the requesting UE sends the message to the base station via the relay UE. According to the invention, high data rate transmission can be achieved even if the requesting UE is distant from the base station, and thus communication quality can be improved, with a reduced cost.