Effect　of　ionic　strength　and　calcium　ions　addition　on　the　molecular　properties　of　humic　acid　(HA),　as　well　as　the　HA　rejection　coefficient　and　fouling　characteristics　of　ultrafiltration　were　investigated　in　this　study.　Experiments　were　performed　with　an　immersed　hollow-fiber　polyvinyl　chloride　membrane　at　a　laboratory　scale.　The　results　indicated　that　both　ionic　strength　and　calcium　ions　could　effectively　reduce　zeta　potential　of　the　HA　solution.　Average　size　of　HA　was　identical　with　various　ionic　strength　(10,　20,　and　40　mM)　or　low　calcium　ion　concentration　(0.5　mM),　but　a　much　larger　HA　size　was　obtained　with　higher　calcium　ion　concentrations　(1.0　mM　or　2.0　mM)　for　the　complexation　interaction　between　HA　molecules.　The　ionic　strength　significantly　reduced　the　HA　removal,　and　somewhat　mitigated　the　membrane　fouling　for　the　reduction　of　electrostatic　repulsive　forces　between　HA　molecules　and　the　membrane　surface.　At　a　low　calcium　ion　concentration,　the　surface　complexation　dominated　the　fouling　behavior　of　the　process,　and　led　to　an　increase　of　cake　fouling　resistance.　However,　at　higher　calcium　ion　concentrations,　the　bulk　complexation　dominated　the　interaction　fouling　behavior　of　the　process,　leading　to　a　reduction　of　both　cake　fouling　and　pore　blocking　for　the　larger　size　aggregations.