In　a　coagulation-flocculation　process,　optimal　separation　of　the　resultant　aggregates　plays　a　decisive　role　on　coagulation　performance　and　provides　a　lower　burden　for　subsequent　treatment　units.　This　separation　highly　depends　on　the　stability　of　the　micro-flocs　formed　during　the　initial,　rapid　stage　of　coagulation.　In　this　work,　a　two-stage　addition　of　aluminium　sulphate　(alum,　Al-2(SO4)(3))　was　employed　by　adding　0.04　and　0.08　mM　Al-2(SO4)(3)　at　the　beginning　and　the　end　of　rapid　mixing,　respectively.　The　coagulation　performance　and　floc　characteristics　were　compared　to　conventional　single　addition　with　the　same　total　coagulant　dosage,　and　the　effects　of　variable　rapid-mixing　speeds　(160-850　rpm)　and　duration　time　(10-120　s)　were　investigated.　The　results　showed　that　the　residual　turbidity　of　two-stage　coagulant　addition　was　85.1%　lower　than　single　addition　when　applied　at　a　mixing　speed　of　580　rpm　and　a　duration　time　of　120　s.　The　underlying　coagulation　mechanism　revealed　that　the　two-step　addition　more　effectively　neutralized　colloids　and　formed　larger　aggregates　that　settled　better　and　could　more　easily　be　removed.　Moreover,　the　aggregates　were　less　firm,　which　was　attributed　to　different　interactions　among　the　micro-flocs,　the　second　addition　of　Al-2(SO4)(3)　and　destabilized　colloids.　The　present　work　provides　data　to　broaden　the　window　of　rapid-mixing　environments　for　more　effective　coagulation.