Disclosed is a method of realizing a link access control protocol for IP M/B packet transmission in a mobile communication network. The method of the invention comprises the steps of: receiving by an LAC of a BTS/BSC an Internet IP M/B packet transferred via a BSC/PCF from an Internet host and a multicasting server, storing the Internet IP M/B packet into an internal buffer, and segmenting the Internet IP M/B packet into a radio packet frame size; adding sequence numbers to the segmented radio packet frames and transmitting a CIBP SDU into a mobile terminal; and assembling, in the mobile terminal, the CIBP SDU for the received radio packet frames and forming the IP M/B packet, and transferring the formed IP M/B packet into an upper layer.