Abstract: The GPS displacement monitoring system is a new method for measuring ground surface displacements using Global Positioning System (GPS) technology. It can measure the three-dimensional displacements of multiple measurement points on the ground surface automatically and continuously. The present study applies the system for measuring the exterior deformation of two fill dams, namely, the Majimegawa Dam, a 21.9-meter-high earth-fill dam located in Yamaguchi Prefecture, Japan, and the Taiho Sub-dam, a 66.0-meter high rock-fill dam located in Okinawa Prefecture, Japan.This paper reports the observational results and verifies the effectiveness of the displacement monitoring system. From the observational results, we find that the GPS-based displacement measurement system was able to detect with high accuracy the very small movements that followed the changes in the water level of the reservoir and the compression of the dam core.