The problem of loading effect on high precision GPS observations is discussed in this paper. According to the Green's approach in theory of mass load on Earth's surface, the vertical displacements at some Chinese IGS stations due to mass load, including atmosphere, nontidal ocean and land water, are estimated. The atmosphere loading effects on different stations are of the similar characteristic, which has the magnitude of 20mm; the nontide ocean loading effects are accountable only at the costal stations, which have magnitude of 10mm; and the landwater loading effects are various at different stations, which have magnitude of 20mm. And the total effects of the three sources have obvious seasonal variation with magnitudes of the order of centimeter, which are corresponding to the mass load and comparable with the observations obtained by GPS. Especially for stations of Fangshan and Wulumuqi, the seasonal variations in GPS time series primarily result from mass load. Furthermore, although the stations of Kunming and Lasha locate in active plate motion area, the seasonal variations mostly attribute to surface mass load. Therefore, the GPS observations with the loading effect excluded will contribute more to investigation on the characteristic of crustal movement.