Membrane fouling in BSA/dextran binary suspension cross-flow microfiltration under various operating conditions is studied. The mechanisms for membrane fouling based on the SEM and CSLM observations under various suspension concentrations are proposed. BSA aggregates deposit onto the membrane surface while dextran molecules adsorb into the membrane pores, resulting in filtration resistances. The BSA deposition is mainly determined by the drag forces exerted on BSA aggregates on the membrane surface. Dextran adsorption is affected by the dextran concentration and applied pressure. The concentration effects on the filtration flux and filtration resistance are also discussed. The filtration fluxes for BSA/dextran mixtures are located between those for two pure substances and decrease with the increase in either the BSA or dextran concentration. However, the dextran concentration impact is more significant because the filtration resistance caused by membrane blocking is much higher than that caused by cake formation in most conditions. The filtration resistances due to concentration polarization, cake formation and membrane internal fouling can be estimated directly from the operating conditions using the semi-theoretical methods proposed in this study.