GPS campaigns have been conducted since 1988 in the Sagami bay area, south Kanto, Japan by the Japanese universities consortium for GPS research . During the early period of observations, precision of GPS observations in the Sagami bay area was very lowbecause the number of observable satellites was very poor . Recently, precision of GPS observations in this area has been much improved year by year since the number of observable satellites increased. We study precision of GPS observation with the aid of free network adjustment. Standard error spherical radius of position for 1988 and 1992 are 3.1-6.3 cm, 0.4-0.5 cm respectively . That is, the precison of GPS observations for 1992 is one order better than that for 1988.