A method of performing user authentication of a mobile station by proxy in a communication system is provided. The method includes receiving an authentication request, which requests authentication of a user, from an Internet service provider having received information indicating that the mobile station can perform authentication based on only the mobile station's own information, requesting the user to provide security identification information for authentication, receiving the security identification information input by the user, authenticating the user by determining whether the security identification information is valid information, through security-requiring information managed by the mobile station, and transmitting an authentication result to the Internet service provider and receiving an authorized authentication result from the Internet service provider and providing a service according to the authorized authentication result to the user.