Alkaline flocculation is low-cost,low energy consumption and non-toxic to microalgal cells.In this paper,a response surface methodology(RSM)was used to evaluate the effects of pH and microalgal biomass concentration(BC)on alkaline flocculating activity for harvesting of three strains of typical energy microalgae,freshwater green algae: Chlorella zofingiensis G1; Scenedesmus sp.G4 and one marine diatom Phaeodactylum tricornutum #863.The pH value and BC were in range of 9.0~12.0 and 0.20~3.0 g/L,respectively.Response test matrix models were constructed according to the central composite design.Furthermore,results also showed that the addition of synthetic ocean water(SOW)in low algae concentration to the freshwater strains culture can increase the flocculating activity significantly,which maybe provide the technical support for the real seawater utilization.