Location based service is an indispensable part of today's mobile era. While it brings a lot of benefits to people, the breach to individual location privacy is always a concern and impedes the smooth development of location based service. A user can be easily tracked once she subscribes or uses the service from an untrusted location based service server. In this paper, we try to address this problem by proposing a secure and efficient location based service system. In our system, a user does not leak any of her location information while she can still obtain the desired information associated with the location. We propose a novel method to map a user's current location to the index of the information stored in the location based service server. We demonstrated the efficiency of our system through simulations.