In the pervasive computing environment, the users have to provide certain authentication information to the services provider to get the services they need, but the authentication information often contains the privacy which the users do not want to be leaked. In order to protect this privacy, this paper proposed a privacy preserving policy based on rough set theory in authentication for pervasive computing: the users expand the authentication information to a rough set and then provide the rough set to the services provider, the services provider extracts the users’ true authentication information from the rough set according to the pervasive preserving policy and then authenticate the users’ service request. The privacy preserving policy makes use of the uncertainty of rough set and can effectively prevent the leakage of user’s privacy.