Flocculation is known one of the effective methodsfor harvesting microalgae. This study was aimed to optimizethe flocculation condition for decreasing the amounts offlocculant and obtaining the highest yield of algal biomass. Toachieve this goal, it was optimized the flocculant concentration,reaction pH and the concentration of cell density for harvestusing response surface methodology (RSM). The flocculationof microalgae, Spirogyra varians, was carried out usinginorganic flocculant polyaluminium chloride. By the RSM result,the optimal flocculation condition was calculated 5 ppmof polyaluminum chloride, pH 7.5 and 0.33 of optical celldensity at OD640. The obtained recovery yield of S. varianswas 97.6% at the optimal condition.