A one-pass authentication mechanism and system for heterogeneous networks are provided. The mechanism comprises authenticating a user based on an authentication key and an authentication algorithm in response to a request of the user to register a first network, wherein the authentication key and the authentication algorithm are associated with a first user identity for the first network and a second user identity for a second network; and if the authentication is successful, then comparing the first user identity retrieved from an authentication database through the second user identity provided by the user to the first user identity provided by the user in the authentication, in response to a request of the user to register the second network, and setting up security associations between the user and the second network if the retrieved first user identity matches the first user identity provided by the user.