A method and apparatus of releasing a radio bearer in a wireless communication system is provided. After determining to release a radio link control (RLC) entity and a packet data convergence protocol (PDCP) entity, the RLC entity is released after delivering an RLC service data unit (SDU) extracted according to a release request of the RLC entity to the PDCP entity. A first PDCP SDU obtained by processing the RLC SDU is stored in a reception buffer at the PDCP entity. The PDCP entity is released after delivering the first PDCP SDU stored in the reception buffer to a higher layer.