The present invention relates to a wireless communication system. More specifically, the present invention relates to a method and a device for transmitting a power headroom reporting in wireless communication system, the method comprising: generating a PHR MAC CE including PH information per BWP within a serving cell, in a state that the UE is configured with one or more BWPs per serving cell; and transmitting the PHR MAC CE, wherein the PHR MAC CE includes a BWP bitmap indicating presence of the PH information per BWP within the serving cell, wherein the BWP bitmap is present when the serving cell is activated. The UE is capable of communicating with at least one of another UE, a UE related to an autonomous driving vehicle, a base station or a network.