Apparatus and method for sending a multicast packet in a mobile digital broadcast system are provided. The mobile digital broadcast system includes a broadcaster for indicating the designated recipient in an Internet Protocol (IP) header of a multicast packet and including a temporary IP address of the recipient in the IP header when multicasting data of a designated recipient; a receiver for providing the received multicast packet to an application program for outputting a digital broadcasting when the receiver joins a group of the multicast packet and the recipient is designated to the receiver upon receiving the multicast packet; and an IP allocating server for allocating a temporary IP address when the receiver requests to allocate the temporary IP address, storing the allocated temporary IP address and ID information of the receiver, and sending the allocated temporary IP address and the ID information of the receiver to the broadcaster.