Abstract: The present paper introduced a broad outline of the diversity of protozoa on the globe , the position in the biology and the relation to the human being. Considering that the protozoa in freshwater as an aggregate group ecologically , its diversity and stress could be discussed. The stress resulting from anthropogenic sources in an eutrophication lake —Donghu Lake in a period of half a century resulted in a ten2fold increase of protozoan density and a succession of dominant species. The industrial pollution from the bank of the Three2Gorges Area of the Yangtze River damaged at least 70 % species of protozoans. In the largest tributary of the Yangtze River , the Hanjiang River , a resource of drinking water , the protozoan diversity index indicated that water quality was in a critical status. In the unexploited Wuling Mountains Area in southwestern China , the protozoan diversity demonstrated that the aquatic ecosystem in this area was healthy and stable.