A method and system for transmitting a short message to a called party's Internet phone use an Internet phone according to the H.323 multimedia communication protocol. The invention provides a message transmission system between a short message transmission server and a gatekeeper. The gatekeeper controls setup of a call and user registration or cancellation in the Internet phone. The Internet phone is optionally connected to the gatekeeper, and has a short message transmission module for transmitting, via a predetermined port, the short message including information corresponding to a telephone number of the called party's Internet phone. The short message transmission server serves as an H.323 terminal registered in the gatekeeper, and is constructed to transmit, to the gatekeeper, the information relating to the called party's Internet phone incorporated in the corresponding short message so as to obtain an Internet protocol (IP) address of the called party's Internet phone, and to transmit the short message to the IP address of the called party's Internet phone. The invention allows a user of the Internet phone to transmit the SMS message to the called party's Internet phone using the registration admission and status (RAS) protocol of the H.323 multimedia communication protocol.