Abstract: Based on the temper ature,salinity and depth data measured from 1975 to 1998 in the northern South China Sea,variation of sound velocity of upper water(0~100 m) is discussed.The results show that annual variation of sound velocity of sea water is of importance.Dist ribution of sound velocity of sea water in the surface is notably influenced by water masses.There is a largest horizontal gradient near the continental shelf margin in the northern South China Sea in winter,it gets weaker and weaker from spr ing to summer,to autumn.Distribution of sound velocity of sea water in 50 m depth is complex because of jointaction of circulation and water masses.In spring and summer sound velocity decreases as depth increasing,but in autumn and winter it increases as depth increases in the upper layer and then decreases as depth increasing.Variation of monthly-averaged sound velocity looks like a annual wave variation and is similar to variation o f temper ature.Variation amplitude decreases as depth increasies and its phase in the surface is almost opposed to that in 100 m depth.