Abstract: Effects of concentration of atrazine on growth of Scenedesmus quadricauda and Microcystis aeruginosa cultured in MA medium were studied by determining growth rate and content of chlorophyll a in laboratory.With the maximum specific growth rate,expressed by count of algal cells as evaluation index,the two algae were compared in sensitivity to atrazine.When low in concentration,atrazine stimulated growth of Scenedesmus quadricauda and Microcystis aeruginosa;but when high,it demonstrated a reverse effect.And its stimulating effect was greater on Scenedesmus quadricauda than on Microcystis aeruginosa,whereas its inhibiting effect showed the other way.