Disclosed are a communication technique for merging IoT technology with a 5G communication system for supporting a data transmission rate higher than that of a 4G system and a system therefor. The present disclosure can be applied to intelligent services (for example, smart home, smart building, smart city, smart car or connected car, healthcare, digital education, retail, security, and safety related services, and the like) on the basis of 5G communication technology and IoT-related technology. The present invention relates to operations of a terminal and a base station in a mobile communication system. The present invention provides a method by which a terminal re-establishes a PDCP in a wireless communication system, the method comprising the steps of: receiving a message including PDCP re-establishment information on a DRB from a base station in which an RRC connection is set up; checking whether the PDCP re-establishment information includes configuration information of an SDAP layer; and determining whether to perform a PDCP re-establishment procedure for changing the DRB operating as a PDCP of a first system into a PDCP of a second system on the basis of the checking result.