A communication system, a data transmission method thereof, and a user device thereof are provided. The data transmission method includes the following steps. A first transmitting Radio Link Control (Tx RLC) entity of a first base station transmits a first Protocol Data Unit (PDU) to a first receiving Radio Link Control (Rx RLC) entity of a user equipment via a first leg. A second Tx RLC entity of a second base station transmits a second PDU to a second Rx RLC entity of the user equipment via a second leg. If the first Rx RLC entity successfully receives the first PDU or the second Rx RLC entity successfully receives the second PDU, the first Rx RLC entity and the second RLC entity return Acknowledgement (ACK) messages.