In this paper factors affecting the ultrafiltration of mixtures of HA and BSA including concentration, pH, ionic strength, ionic charge and size of hydrated ion were elucidated. In the presence of BSA, higher HA rejection and lower flux were obtained during ultrafiltration. This is due to interactions between HA and BSA. A bond may be established between humic acid and protein molecules. Flux was decreased and HA rejection was increased with increasing the humic acid concentration in constant concentration of BSA. At low concentration, the humic macromolecules pass more easily through the membrane pores due to their structure. The HA rejection depends on the molecular weight cut off (MWCO) of the membrane. Higher rejection is anticipated for membranes with lower MWCO. However cake deposition is the dominant mechanism during ultrafiltration of solutions containing mixtures of HA and BSA using XM50, YM100 and XM300 ultrafiltration membranes. By increasing the pH of solutions containing mixture of HA and BSA, higher flux and lower HA rejection were achieved. At the protein isoelectric point, flux showed a minimum and increased for higher pH values. A higher flux was obtained for solutions with lower ionic strength. At the beginning of the filtration, HA rejection was low for solution with higher ionic strength. However during filtration the trend was conversely changed. In the presence of Ba2+ cation the HA rejection for solutions containing HA and BSA was higher compare to the solutions containing Na+ cation.