A multimedia-messaging-service (MMS) system and the service method thereof are provided. The MMS system integrates the mobility management scheme of a GSM/GPRS/UMTS mobile network and the broadband transmission service of a wireless LAN, and has a user authentication and a location management. The MMS system uses a MMS server to communicate with a wireless LAN, a GSM/GPRS/UMTS mobile network and an Internet for providing the MMS service including user registration, message submission, message forwarding, message notification, message retrieval and location update.