A method of transmitting security data from a source point to a target point in a point-to-multipoint communication system is provided. The method includes the steps of forming an encryption tag field that includes information about an encryption tag type, which indicates whether transmission data is encryption-enabled or encryption-disabled; forming a packet data field which includes encrypted transmission data; forming a first integrity check field indicating the CRC check sum of the encrypted data; forming a length and type field indicating the sum of the lengths of the packet data field and the first integrity check field. Finally, a transmission frame is formed with these formed fields, a source address field indicating the address of the source point, and a destination address field indicating the address of the target point.