Abstract: A user authentication method of authenticating a user on an on-line basis using a user's e-mail address and hardware information is provided. The user authentication method includes the steps of: transmitting a user's authentication client platform hardware information and e-mail address to an authentication server module through an authentication client module installed in the authentication client platform; and the authentication client module determining user authentication according to whether or not the e-mail address and hardware information received from the authentication server module is identical to user's e-mail address and hardware information stored in an authentication database. A user's authentication request and authentication is confirmed through an e-mail in the case that authentication fails in the primary authentication process. The user authentication method performs authentication by using a user's hardware information and e-mail address, their uniqueness of which is verified.