The effects of operating conditions on the separation efficiency of α-amylase employing cross-flow ul-trafiltration unit were investigated. High transmembrane pressure and low rate gave a higher separation ef-ficiency. Severe deactivation of the enzyme during cross-flow ultrafiltration was observed when the transmembrane pressure was high and both flow rate and the enzyme concentration were low. The permeate flow rate could be well predicted by using related model equations, especially for the feed stream of low enzyme activities.