Precipitable Water Vapor (PWV) can be estimated from ground-based GPS observations. In Japan, such GPS PWV can be obtained from a nationwide permanent GPS network which is called GPS Earth Observation NETwork (GEONET). The GEONET is operated by the Geospatial Information Authority of Japan. The number of the GEONET GPS sites is about 1,200. The Japan Meteorological Agency (JMA) has been using the GPS PWV from GEONET in the operational mesoscale data assimilation system since October, 2009. For the global data, the international GNSS Service (IGS) operates a global network of ground-based GPS stations continuously for GPS satellite tracking, and provides GPS observation data via its FTP server. We estimated GPS PWV for each IGS site and assessed the accuracy of the estimated GPS PWV data. For the GPS PWV analysis, we used the GIPSY/OASIS II software package, provided by the Jet Propulsion Laboratory (JPL) of the National Aeronautics and Space Administration (NASA). The result showed that the estimated GPS PWV is close to the one estimated by radiosonde observation. And then, the impact of the GPS PWV data on JMA's global four-dimensional variational data assimilation system was evaluated. It showed that the assimilation of the GPS PWV brought positive impact compared without the GPS PWV in the geopotential height forecast at 500hPa. In this presentation, the details of the results of the experiment will be discussed.