Provided are a method of performing cell reselection and a device supporting the method. According to one embodiment of the present disclosure, the method includes: performing data transmission or data reception while the UE is in RRC IDLE state or RRC INACTIVE state; measuring a serving cell quality; and starting a cell reselection prohibit timer when the quality of the serving cell is better than a threshold, wherein a cell reselection procedure is prohibited while the cell reselection prohibit timer is running.