The GPS displacement monitoring system is a new method that measures the displacements of the ground surface using Global Positioning System (GPS) technology. It can automatically and continuously measure the three-dimensional displacements of multiple measurement points on the ground surface. The present study applied this system to measure the exterior deformation of Majimegawa Dam, a 21.9-meter-high earth-filled dam located in Yamaguchi Prefecture, Japan. This paper verifies the effectiveness of the system through the observational results from a three-year period. Although the displacements of the dam body, from the beginning of the impoundment to the present, have been less than 10mm at the maximum, the GPS displacement monitoring system has detected with high accuracy very small movements following changes in the water level of the reservoir and the compression of the dam core.