In order to improve the efficiency and accuracy of the existing Wind and Structural Health Monitoring System (WASHMS) for the three cable-supported bridges, namely, the Tsing Ma (Suspension) Bridge, the Kap Shui Mun (Cable-Stayed) Bridge and the Ting Kau (Cable-Stayed) Bridge, Global Positioning System (GPS) is introduced to monitor the displacements of the cables (main suspension cables only), the stiffening decks and the bridge-towers. The measured displacement values will be used to calculate relevant motions at center-lines of the stiffening decks and bridge-towers, which will then be used to derive relevant stress status acting on the major bridge components. The GPS here refers to the `NAVSTAR GPS or NAVigation System with Time And Ranging Global Positioning System' which is a satellite-based system that uses a constellation of 24 (currently 27) satellites to determine the accurate coordinates or position of a user/receiver. The GPS applied to monitor the displacements of three cable-supported bridges is known as `Global Positioning System--On-Structure Instrumentation System or GPS-OSIS'. This GPS-OSIS is a real-time monitoring system and made up of five sub-systems, namely, the GPS Sensory System, the Local Data Acquisition System, the Global Data Acquisition System, the GPS Computer System and the Optical Fiber Network System. This paper first introduces the system layout and technical performance requirements of the GPS-OSIS, then briefly discusses the applications of GPS results in structural health monitoring of cable-supported bridges, e.g. the correlation of bridge responses to the effects of wind, temperature and traffic loads and the significance of monitoring geometrical variations and dynamic characteristics.