Disclosed are a radio link control (RLC) entity and a data processing method for the RLC entity. The RLC entity includes a transmission data storing module that stores PDUs corresponding to SDUs transmitted from a first upper layer and outputs the stored PDUs by SDU units, a ciphering module that ciphers the PDUs stored in the transmission data storing module and transmitting the ciphered PDUs to a first RLC entity, a deciphering module that deciphers the ciphered PDUs transmitted from a second RLC entity, and a received data storing module that stores the deciphered PDUs and outputs the PDUs toward a second upper layer in the form of SDU units.