According to an embodiment, a method performed by a first wireless device in a wireless communication system includes: reserving a set of resources comprising at least a first resource for an initial transmission and a second resource for a retransmission; creating a data unit based on the first resource and the second resource; performing the initial transmission of the data unit to a second wireless device by using the first resource; determining whether to reserve a resource for the retransmission of the data unit based on at least one of a congestion level and/or a priority of the data unit; removing the second resource and reserving a third resource as the resource for the retransmission of the data unit; and performing the retransmission of the data unit by using the third resource to the second wireless device.