Property tax provides a significant amount of revenue to local governments. In order to maintain a just tax system, the tax base shall be constantly re-valued and potential tax inequity monitored. Valuation of real properties for tax purposes is in nature a mass appraisal and an method is needed to detect the potential appraisal inequity among properties. Regression analysis has been employed to measure the inequity of property tax and proven to be useful. However, the recent addition of geographic information system (GIS) to the analytical tool box seems to be promising, particularly in processing spatial data. This paper explores the benefits of GIS in evaluating inequity of property tax through its unique functions. With the help of a data set of sale prices and their corresponding assessed values for taxation, the assessment ratio is estimated for individual properties. All these assessment ratios are identified as points in space through address-matching. Central tendency and dispersion of these points that represent assessment ratio are estimated. Through the distribution of assessment ratios, the properties whose ratios are far away from the average figure can be easily identified in location. The spatial data will be further analysed by other GIS functions and hope to uncover more information that is ignored in traditional non-spatial analytical methods.