This paper presents a novel 802.11-based positioning system called the Best Signature Match (BSM) positioning system developed and demonstrated in the National Central University. The BSM positioning system partitions the service area into several regions and assigns each region a signature which contains a sequence of access point IDs with the average corresponding power strength statistics measured in the region. The BSM positioning system locates mobile users via collecting the sensed power strengths of the mobile users to multiple access points and then applies the best match approach to determine the regions in which the mobile users stay. A location database called the region signature database (RSDB) is applied to maintain the power signatures of all the regions. To reduce the RSDB maintenance cost, the BSM positioning system provides a mechanism which can automatically update the RSDB, The feature significantly reduces the RSDB maintenance cost. The experimental results validate that the location information provided by the BSM positioning system assures high correctness of region classification.