在室內定位系統 (Indoor Position System) 的技術發展上，所採用的技術其實相當廣泛，任何無線相關可以達到定位的技術都能夠採用。隨著Wi-Fi無線網路的普及，我們生活周遭到處已經被許多Wi-Fi無線網路訊號覆蓋，而Wi-Fi也已經是現在智慧型手機必備的功能；因此，透過Wi-Fi無線網路訊號的定位方式，是最容易被取得的無線技術。本論文提出以無線基地台 (Access Point, AP) 所使用無線訊號發射功率 (Power Level, PL) 的不同，利用智慧型手機Wi-Fi搜索到的AP、即 (Service Set Identifier, SSID) 此無線網路名稱包含座標和Power Level值，取最近4個座標位置兩兩交集點，並過濾出4個AP可見距離範圍內的交點為合理預期邊界，利用這些焦點所形成的多邊形，取其重心即為定位出所在的位置。模擬的結果顯示其定位誤差在可接受的範圍內。;In the development of Indoor Positioning System, a wide range of technology is universal available. Any wireless related technology which can achieve positioning can be adapted. With Wi-Fi solution becomes ubiquitous it is a vital technology to support, in our day-to-day life. In fact, we are covered by many Wi-Fi signals wherever we are. Along with it being a built-in feature on modern smartphones, therefore Wi-Fi technology is the most commonly obtainable wireless technology. This thesis proposes to use the Power Level (PL) in Wi-Fi Access Point (AP) to support indoor positioning. The APs periodically sends beacons with unique Service Set Identifier (SSID), which carries the coordinates and corresponding PL value. AP broadcasts beacons using different power levels, one at a time. Smartphone can easily collects the visible APs with certain power levels and then find out the 2 intersections of any pair of nearest APs. Using the coordinates of visible APs, we can filter out the available 4 intersections within viewable range. Using these filtered intersection points, we can find out the center point of the polygon, which is the estimated position of the smartphone. Simulation results reveal the derived error of positioning is tolerable.