The effect of oil viscosity on flux in membrane separation process was studied in the stirred batch cell. Membranes used in our experiment were FS-81PP and FSM-2.0PP from Denmark DDS Co. and test solutions were the mixtures of base lube oils. It was found that the relationship between the inverse flux(1/Jv)and the kinematic viscosity of oil(ν) can be shown as 1/Jv=Aν+B where A and B are the characteristic parameters of the membrane under given operation conditions. It was also found that the effect of temperature can be interpreted as the change in oil viscosity.