A method and system for providing a Push-To-Talk (PTT) service in association with a Radio Access Network (RAN) and an Internet Protocol (IP) network are provided. Each of access terminals (ATs) supporting the PTT service is allocated a multicast IP during session setup, and joins in a chat room for a group for the PTT service using a multicast protocol message. Each of access networks (ANs) allocates a shared channel to the RAN to process a group call between ATs in the group if the AT is an AT in the group. A PTT system performs session setup with the AT, allocates information including a multicast IP allocated to the group of the AT during the session setup, and after the session setup, provides the PTT service between the ATs in the group using a routing protocol for the multicast IP in the IP network. Each router configures a multicast tree of the group, and upon receiving transmission data from the AT, transmits the data to the ATs in the group according to the multicast tree.