The present invention relates to a wireless communication system. More specifically, the present invention relates to a method and a device for handling state variables of a PDCP entity in wireless communication system, the method comprising: delivering an PDCP SDU associated with a first COUNT to upper layer and starting, by the PDCP entity, a reordering timer, when an out-of-order PDCP PDU with the first COUNT value is received from lower layer; updating a value of a first state variable to the first COUNT value+1, when the reordering timer starts; and when the reordering timer expires, updating, by the PDCP entity, a value of a second state variable based on a second COUNT value of a first PDCP SDU which has not been delivered to upper layers, among PDCP SDUs with COUNT values which are equal to or larger than the first state variable.