In this study, the extracellular polymeric substance of Scenedesmus quadricauda called EPS-S.q and its bioflocculatoin mechanism were investigated. Results showed EPS-S.q was successfully used as bioflocculating agent for S. quadricauda biomass harvesting and flocculation efficiency of up to 86.7 to S. quadricauda cells could be achieved in presence of Zn2+. EPS-S.q was the flocculating agent for self-flocculating microalga S. quadricauda and bioflocculation mechanism was polymer bridging. The sugar and protein mass fraction of dry EPS-S.q was sugar 56.7 and protein 41. The infrared spectrum further indicated the presence of hydroxyl, carboxyl and amino groups. Moreover, pH decrease induced the flocculation of S. quadricauda and 78.4 of flocculation efficiency was the highest at pH 3. In addition, chemical flocculant FeCl3 was efficiently used for S. quadricauda harvesting and up to 96.8 of flocculation efficiency could be achieved for S. quadricauda culture with biomass concentration 0.21�0.39 g L�1 at pH 7. © 2016