Many ultrafiltration-related studies have emphasized fouling mechanisms, but few works have been done on resistance modeling. This study investigated the temporal variation of different resistances including membrane intrinsic resistance, fouling resistance, and concentration polarization resistance, based on a well established resistance-in-series model. The various resistances were determined at an early stage of ultrafiltration operation. During the initial operation period, the total filtration resistance for ultrafiltration of humic substance solution ranged from 1.9x10(9) to 2.2x 10(9) Pa.s/m. The principal resistance of ultrafiltration is from intrinsic membrane resistance, accounting for more than half of the total resistance (56-85%).