The present invention relates to a wireless communication system. More specifically, the present invention relates to a method and a device for handling data lossless in a UM RLC entity in wireless communication system, the method comprising: receiving a Radio Resource Control (RRC) message for releasing or reestablishing of a Unacknowledged Mode (UM) Radio Link Control (RLC) entity; transmitting, by the UM RLC entity to a Packet Data Convergence Protocol (PDCP) entity associated with the UM RLC entity, information for transmission status of RLC Service Data Units (SDUs) stored in the UM RLC entity; and discarding all RLC SDUs and Unacknowledged Mode Data (UMD) Protocol Data Units (PDUs) stored in the UM RLC entity.