Methods and apparatuses for transmitting data duplication in a wireless communication system are disclosed herein. In one method for a user equipment (UE) configured with data duplication. The UE duplicates a data unit into a first data unit and a second data unit. The UE constructs a second Radio Link Control (RLC) Packet Data Unit (PDU) used for transmitting the second data unit. The second data unit is a second RLC Service Data Unit (SDU). The UE receives a control command from a network node. The control command is used to deactivate the data duplication. The UE discards the second RLC PDU if the second RLC PDU includes the second RLC SDU.