An embodiment of the present disclosure provides a method for performing sidelink communication by a first device. The method may comprise the steps of reserving periodic transmission resources on the basis of a first priority value; generating a MAC PDU related to a second priority value; on the basis of the second priority value, determining whether to reselect a first SL resource among the periodic transmission resources; and on the basis of the determination on whether to reselect the first SL resource, transmitting the MAC PDU and SCI including the second priority value to a second device, wherein the first SL resource is at least one SL resource after the MAC PDU is generated.