A method and apparatus for detecting a security error at a PDCP layer of an LTE (Long Term Evolution) system, a mobile communication system, are disclosed.Conditions for determining a security failure are defined. A receiving side PDCP layer determines whether HFN de-synchronization, namely, a security failure, has occurred by using particular conditions (namely, conditions for determining the security failure). If it is determined that the security failure has occurred, the receiving side PDCP layer informs an RRC layer to re-establish an RB or perform a PDCP RESET procedure to re-set security configuration of a transmitting side and the receiving side.