► The Scenedesmus sp. discussed in this study was cultivated in an open-pond system. ► Optimal conditions of six flocculants for harvesting Scenedesmus sp. were discussed. ► Optimal pH to each flocculant was found for achieving high flocculation efficiency. ► Increasing algal biomass concentration will increase the flocculant dosage needed. The effects of culture medium pH, flocculant type (FeCl3, Al2(SO4)3, Alum, Ca(OH)2, chitosan, polyacrylamide), dosage and sedimental time on flocculation efficiency of harvesting Scenedesmus sp. cultivated in an open-pond system were investigated. Meanwhile, the relation between initial biomass concentration and the flocculant dosage needed was also investigated. The results from this work indicated that the flocculation efficiency achieved 97.4% after 10min of sedimentation when the pH was adjusted to be 11.5, without adding flocculants. FeCl3 and chitosan showed a good flocculation efficiency at dosage of 0.15 and 0.08g/L, respectively without pH adjustment. The flocculation efficiency increased from 49.74% to 90.63% when the final medium pH was adjusted to 6 after adding 0.1g/L Alum. An increment from 68.18% to 92.84% was observed after adding 0.1g/L Al2(SO4)3 followed by pH adjustment. Finally, the most suitable flocculation method was discussed in this paper.