The area of lakes in Qinghai-Tibetan Plateau is one of the largest and the highest lake district in the world, most of which lie on the north Tibetan Plateau (Qiang tang Plateau)with an average altitude of more than 5000m a.s.l.In the summers of 1997 and 1998, we investigated the hydrog raphic features of Sêrling Co, north Tibetan Plateau.The hourly maximum temperature of epilimnion occured at 4:30 a.m.and the minimum w as at 7:30 p.m.The maximum temperature of epilimnic shallow region could reach at 21.2℃ and the minimum was 11.0℃ in June.The range of daily mean temperature of epilimnion was only 5.6℃ and was larger than the difference of that w as co rrespo ndingly tested among different regions in Sêrling Co.The water pH value of epilimnio n in the lake varied from 9.19 to 9.66.However, the range of hourly pH value in shallow region was only 0.15 and the maximum occured in the early morning, w hich differs from other lakes such as the lowland lakes in east China.The dissolved oxygen v aried f rom 4.62 mg/L to 5.12 mg/L and the changes among dif ferent deep regions w ere only 0.03-0.35 mg/L in Sürling Co.Hourly gradient of dissolved oxygen of epilimnion varied f rom 4.85 mg/L to 5.59 mg/L in Jiang dongrurui (aquene of Sürling Co), 4.72-5.35mg/L in the mouth of Boquzangbu and 4.54-5.93 mg/L in the low er reaches of Boquzangbu.The hourly minimum of dissolved o xygen in Jiangdong rurui and Boquzangbu occured at 4:30 p.m., and in the outlet of Boquzangbu at 7:30 a.m.The times that maximum dissolved oxygen observed were so different and occurs respectively at 10:30 p.m., 7:30 p.m.and 10:30 a.m.in the regions above-mentioned respectively, which proved that the dissolved oxygen could be influenced by water temperature and hydrobiological activities.The hourly salinity of epilimnion in the lake varied f rom 1.6-2.2% and the maximum occured at 4:30 to 7:30 a.m.In the inland water bodies of north Tibetan Plateau, i t is obvious that there is a concentrical tendency of salinity from st reams to lakes and from running water to still water.The dist ribution of fishes is correlative wi th the salinity of waters.Consequently, fishes disappear and are replaced by salinella with the increasing salini ty in water bodies.