The Global Positioning System (GPS) is a worldwide radio-navigation system widely used for tracking tasks in transportation and other fields. In an urban area, where there is a high density of population and transportation activity, many kinds of vehicles, such as buses, taxis, police cars, and vehicles with Hazardous Materials (HAZMAT), need to be accurately tracked in real-time for fleet management or security purposes. Although GPS is excellent in its availability, scope, and precision under ideal circumstances, such as in the air, it does not work well when the receiver is under or near canopies such as trees, high-rise buildings, or tunnels. One effective solution is to integrate the GPS with other tracking systems, such as the Inertial Navigation System (INS). INS relies on no external references after it is initialized and aligned, but its positioning error increases without limit as time goes by. This paper describes our research findings on the performances of a GPS/INS Integrated System and a GPS for vehicle tracking in urban areas. Performance is measured by both positioning accuracy and location data update frequency in our study. Our analysis using the field-test data shows that the GPS/INS Integrated System has much smaller location error than the GPS alone and guarantees the location data update frequency. The GPS/INS Integrated System is a reliable tracking technology for accurately tracking vehicles in urban areas.