A method for ciphering MBMS (Multimedia Broadcast/Multicast Service) data in a wired/wireless communication system is provided. The MBMS service enables the same service or desired services to be provided to a number of users. In the ciphering method, a ciphering key for encoding MBMS data is previously provided to an authorized UE, and the authorized UE is informed of when MBMS data encoded using the ciphering key is to be transmitted, thereby preventing unauthorized UEs from illegally receiving the MBMS service.