The present invention relates to a method for performing uplink transmissions by a user equipment in a wireless communication system. In particular, the method includes the steps of: transmitting a data unit to a network using a configured uplink grant; receiving an uplink grant for retransmission of the data unit from the network; determining a retransmission timing using a parameter related to a retransmission delay; and retransmitting the data unit to the network using the uplink grant for retransmission on the determined retransmission timing. Especially, the configured uplink grant is shared by the user equipment and another user equipment, and the parameter indicates at least one instance of retransmission delay.