Disclosed is a radio (wireless) communication system providing a radio communication service and a terminal, and more particularly, to a method of delivering a continuously and/or consecutively received Packet Data Convergence Protocol (PDCP) Service Data units (SDUs) to an upper layer immediately if a PDCP entity receives the PDCP SDUs during a process of a RLC re-establishment within an Evolved Universal Mobile Telecommunications System (E-UMTS) that has evolved from a Universal Mobile Telecommunications System (UMTS) or a Long Term Evolution (LTE) system.