A semi-empirical model has been developed in ultrafiltration of huanggi (Radix astragulus) extracts. Three major fouling resistances were quantified with transmembrane pressure (TMP) ranged from 0. 4 to 0. 8 bar and process time lasted for 120 min. Adsorption grew fast at the initial 15 min and its contribution was more significant at 0. 4 bar, nearly 50%. Pore blocking almost kept the same values of 1. 28, 1. 84, and 2. 39 at pressures of 0. 4, 0. 6, and 0. 8 bar, respectively. Cake layer grew linearly at 0. 4 bar but it increased rapidly as the TMP arose. With the TMP increased, the contribution of cake layer became more significant, almost 75% at 0. 8 bar which was triple of that at 0. 4 bar. Prediction of flux decline fitted quite well with the experimental data, all within 5% errors. It demonstrates that adsorption, pore blocking, and cake layer are the main mechanisms for membrane fouling during the process. ; Department of Applied Biology and Chemical Technology