In this study, lactose was hydrolyzed using β-galactosidase enzyme in a continuous stirred tank -Ultrafiltration (CSTR-UF) to produce galactose and glucose. The UF membranes of Molecular Weight Cut of 3 kDa of regenerated cellulose material were used to separate enzyme from products. Experiments were performed with 0.139 molar aqueous solution of lactose as feed. The effect of operating pressure ranging between 2 and 5 bar and time on rejection and permeate flux were studied. Results showed that the UF membrane rejects the enzyme completely. Also according to experimental data lactose concentration in permeate decrease with time due to concentration polarization and hydrolysis. It was found that rejection factor of lactose increases from 33 to77%, with time from 5 to 85 min.