Abstract: The treatment effects of Effective Microorganisms (EM) on typical algae-type eutrophical water from Nan Pond are systematically studied in preliminary test. The results indicate that algae biomass in the water can be obviously decreased by casting EM into the vessel with V(EM)∶(V(source water)) ratio of (1∶10 000) and by intermittent aerating for 8～9 d; and that the effluent water quality treated by EM can approach Class Ⅳ of national surface water standard as the removal rate of chlorophyll a, TN, TP and CODCr reaches respectively 90.49%, 45.25%, 55.48% and 82.37%. The pH and conductivity of the treated sample can change with different rules and thus should be two separated factor systems impacting the treatment effect. The EM can regulate the nitric and phosphoric nutriment level and change the correlation between algae biomass and TN/TP ratio and thus becomes the main factor restricting algae growth in the water.