The performance of membrane-enzyme reactor with cyclic ultrafiltration is investigated using the CSTR model proposed by Kim and Chang（1983a）. The cyclic ultrafiltration is induced by a pulsatile inflow and constant outflow having the same flow rate as the mean value of the inflow. The behavior of this reactor is numerically and analytically examined. In addition, overall effectiveness is defined in order to investigate the availability of the soluble enzyme in the membrane reactor.