With the rapid increase of water contamination, membrane separation technology and their corresponding molecular weight cut-off (MWCO) evaluation method become more necessary. In this study, Panax notoginseng saponins was used as a new standard marker to determinate ultrafiltration (UF) membrane MWCOs, series of Millipore membranes were selected as control group to analyze and calculate the relationship between retention rate and MWCOs with exponential or logarithmic equation. A new and convenient method was provided for determining the membrane MWCO by modeling analysis retention rate with MWCOs, and the regression coefficients >= 0.990. The feasibility and practicability of established method was verified by different manufactures' membrane and dextrans. In the detection progress, as the main ingredient of Panax notoginseng saponins, Notoginsenoside R-1, Ginsenoside Rg(1), Ginsenoside Rb-1 and Ginsenoside Rd with different surface activity, the MWCO range of UF membranes can be divided into two zones mainly due to the retention rate difference among Notoginsenoside R1, Ginsenoside Rg1, Ginsenoside Rb1 and Ginsenoside Rd. Zone I, 1000-10000; and Zone II, 10000-100000. Thus, the new method would be helpful to improve the applicability of UF membrane in separation technology. (C) 2016 The Chemical Industry and Engineering Society of China, and Chemical Industry Press. All rights reserved.