Flux-decline in ultrafiltration is studied on a tubular membrane with dextran solution. It is found that even at the onset of the process, the flux is not only governed by the osmotic pressure. Flux reduction is caused by the osmotic pressure and the adsorption resistance simultaneously. With the development of concentration polarisation layer, both the adsorbed layer resistance and the osmotic pressure at the membrane wall increase. Adsorbed layer resistance is found to be a linear function of the wall concentration.