Abstract: Before and after the Haicheng earthquake of magnitude 7.3 which occurred on Feb. 4, 1975, five repeated gravimeter surveys were carried out, three before and two after the earthquake, along a northwest-southeast profile of about 250 km in length not far on the west of the epicenter. The mean-square error of the measurements of the gravity differences between two consecutive points on the profile is less than 40 Gal. From June, 1972 to May, 1973, within a period of about one year, the results of three surveys indicated a clear decrease of gravity values at points on the southeastern portion of the profile, amounting to as much as 352 Gal. After the earthquake, the fourth survey which was carried out on March, 1975, revealed that the gravity values had recovered to the levels at the first survey and continued to increase as was shown by a fifth survey carried out in July of the same year.Variations of gravity were also observed before and after the Tangshan earthquake of magnitude 7.8 which occurred on July 28, 1976, but in this case, gravity was increasing before the earthquake instead of decreasing. Along an east-west profile of about 270km in length and not far on the north of the epicenter, two gravity surveys were made before and two after the earthquake. The results showed that after the main shock, the gravity values of the whole profile, especially at those points closer to Tangshan, tended to return gradually to their values of the first survey before the earthquake.Prom these results, there seems to be a close relationship between these gravity variations and the occurrences of earthquakes. Based on results of repeated levelling work done in these regions, the estimated amount of gravity change caused by the change of elevation of the ground surface is far too small to account for the observed value. We therefore speculate that some big earthquakes might be associated with some sort of mass migration under the ground, within the crust or in the upper mantle. This migration would cause a large part of the gravity variation observed. We have made a theoretical analysis of this effect and attempted to get some estimate of the rate of this mass migration, even though we are not yet clear about the physics of it.