Abstract: Through investigations from March to June 1998 on the phytoplankton in waters of biological-controlled, ordinary rice fields,112 species of phytoplankton were found,of which, Bacillariophyta,Euglenophyta and Chlorophyta were dominant.The comparison of the species in two rice fields showed that in biological-controlled rice field,there were more species of Euglenophyta ,with 5 most dominant species,i.e.,2 of Euglenophyta ,2 of Bacillariophyta, 1 of Chlorophyta .In ordinary rice field,there were more Bacillariophyta species,in which,5 most dominant species belonged to Bacillariophyta ,except Scenedesmus bijuga to Chlorophyta .The biodiversities of phytoplankton, their evennesses were also analyzed with the period from the seedlings being planted to the rice fields being dried,showing that a little increase in their biodiversity mainly caused by the increase of species number, an evident decrease in their evennesses.