Over the last few years, wireless LAN and location-based service has been a hot subject of controversy. In this paper, we propose a Bayesian-based location estimation system and a location-based application using existing wireless access points at a college campus. Our system uses signal strength of access points and a Bayesian inference to estimate a user?s location. Using this pre-observation model-based system and considering signal strength direction, a user can construct a location estimation system easily and obtain good location estimation accuracy. Furthermore, our system allows users to know his/her location whenever they want without beacons searched by a system administrator or an unknown person. We implemented the system and exemplified its effectiveness with experiments at our campus. Using this system, we also propose a location-based presence service system. By using this presence system, one can get the location of friends automatically.