The　interaction　between　polysaccharides　and　proteins　has　attracted　great　interests　and　the　phase　behavior　and　colloidal　properties　of　complexes　between　chitosan,　a　positively　charged　polysaccharide,　and　proteins　still　need　careful　investigation.　Here,　we　use　chitosan/casein　as　model　system　and　monitor　their　complex　size,　zeta-potential,　light　scattering　intensity　and　phase　behavior　as　functions　of　chitosan　deacetylation　degree,　pH　and　ionic　strength.　Chitosan　flocculates　casein　at　low　polysaccharide　concentrations,　while　stabilizes　casein　at　moderate　to　high　concentrations.　Such　flocculation　and　stabilization　ability　are　dependent　on　the　linear　charge　density　of　chitosan,　where　high　charge　density　chitosan　saturates　casein　surface　charge　and　stabilizes　the　complexes　at　lower　polysaccharide　concentration.　The　complexes　are　stable　at　acidic　pH　where　chitosan　is　highly　protonated　and　the　addition　of　NaCl　leads　to　the　reduction　in　both　size　and　zeta-potential　and　facilitates　complex　aggregation.　The　phase　maps　are　constructed　to　provide　guidelines　for　further　studies　on　chitosan/casein　and　other　chitosan/protein　complexes.