Abstract: Melamine is an organic nitrogen heterocyclic compound. It becomes a big problem to the health of people and pets because it is an additive to pet-foods and milks for illegality profits. If melamine releases into the environment, it maybe disturb water ecosystem balance by direct toxic effects on plants and animals, so it should meet the requirements for hazard assessment. Planktonic algae have fundamental importance in proper structure and function of the whole ecosystem, it plays an essential role in the water environment. Moreover, planktonic algae are sensitive indicators for testing different effects of substances discharged into the water. Therefore, the investigations on the influence of toxic substance on unicellular algae have been generally accepted. The purpose of this test is to determine the effects of melamine on the growth of fresh water microalgae (green algae). Exponentially growing test algae were exposed to the melamine in batch cultures over a period of normally 72h, growth inhibition were quantified from measurements of the algal biomass density as a function of time. In the algae reproduction inhibition test, the 94-3000 mg/L concentrations of melamine were applied, melamine was examined to determine the effect concentration (EC) and EC50 (0-72h) values calculated concentrations that would inhibit growth by 50% as compared to the control treatment, they wEre estimated based on growth curve area (EbC50) and growth rate (ErC50). Toxic effects of melamine to algae were determined in an algal growth inhibition test with Pseudokirchneriella subcapitata and Scenedesmus obliquus. The results indicated that the growth of two species was inhibited in different degree by melamine; the higher the concentration used, the greater the toxic effect; the toxic influence of melamine to S. obliquus increased with exposure duration. EbC50 values for 72h of melamine on inhibition of P. subcapitata and S. obliquus growth were 537.67 and 485.17 mg/L respectively, it appeared to be a low harmful substance. Two algae species growth rates were decreased with an increase of melamine concentration; according to estimated EC20 and EC10 values, it concluded that S. obliquus was more susceptive to melamine as compared to P. subcapitata, and appeared to be a clearly creeping effect.