Poly-gamma-glutamic　acid　(gamma-PGA)　broth　was　found　to　be　a　good　bioflocculant　of　microalgae.　However,　the　mechanism　governing　this　bioflocculation　process　is　not　fully　understood.　In　this　study,　Zeta　potential　measurement,　microscopy　examination　and　classical　DLVO　theory　(named　after　Derjaguin,　Landau,　Verwey　and　Overbeek)　analysis　were　used　to　explore　the　flocculability　of　microalgae　induced　by　bacterial　gamma-PGA　broth　bioflocculant.　Microalgae　flocculation　could　be　significantly　improved　by　modifying　ionic　strength　of　the　microalgae　suspension,　lowering　pH　value　and　bioflocculant　addition　due　to　the　stronger　attraction　interactions　between　microalgal　cells.　In　the　present　study,　both　the　pH　reduction　with　the　bioflocculant　induced　better　the　flocculation　process　compared　to　the　modification　of　ionic　strength　of　the　microalgae　suspension　in　the　presence　of　the　bioflocculant　The　DLVO　theory　indicated　that　when　the　bioflocculant　was　introduced,　the　total　interaction　energy　decreased　sharply,　resulting　in　higher　flocculation　efficiency　(＞96　%)　at　a　separation　distance　of　5　nm　and　shorter　settling　time　(from　2　h　to　10　min)　compared　with　that　obtained　only　by　reducing　the　initial　pH.　The　microalgae　interaction　energy　was　found　to　be　dependent　on　the　Zeta　potential.　This　study　provided　a　detailed　interpretation　of　conceivable　mechanism　of　microalgae　bioflocculation　by　gamma-PGA　broth.　(C)　2015　Elsevier　B.V.　All　rights　reserved.