A system and method for providing an instant messaging (IM) service via a Code Division Multiple Access (CDMA) mobile network is provided. A Session Initiation Protocol (SIP) server is connected to the mobile network and an instant messaging service (IM) server via the Internet. The SIP server performs a proxy function for the mobile network allowing the mobile terminal connected to the Internet via the network to receive an instant messaging service from the IM server. The SIP server relays a SIP message, including an instruction word, an instant message or a processed result for an instant messaging service, between the IM server and the terminal. An additional call control according to the Internet protocol in a system is performed, which makes it possible for a user connected to the Internet via the mobile network to communicate instant messages with selected buddies or colleagues.