An optimized model is developed to minimize the process time for the continuous feed diafiltration process under constant concentration ratio of macrosolute, at the same time meeting the requirements of microsolute yield. The optimal functions of minimum process time have been presented under the conditions of constant overall concentration ratio gamma and two steps diafiltration process (gamma = c, without pre-concentration step), where the membrane permeation flux is estimated by the limiting flux model. Based on the model, the optimal control strategies are formulated and figured out in a practical clear way. The strategies can meet the requirements of yield, concentration, and the parameters can be controlled properly. In this case, the model may be used easily both in academic and industrial ways. (C) 2014 Elsevier B.V. All rights reserved.