The present invention relates to a wireless communication system. More specifically, the present invention relates to a method and a device for operating a new entity for LTE-WLAN aggregation system, the method comprising: receiving, by a LTE-WLAN Aggregation Adaptation Protocol (LWAAP) entity, an LWAAP Service Data Unit (SDU) from an upper layer entity; generating, by the LWAAP entity, a LWAAP Protocol Data Unit (PDU) that includes the LWAAP SDU and an identifier identifying a radio bearer to which the upper layer entity belongs; and submitting the LWAAP PDU to a lower layer entity.