Predicting equations for the permeate flux of membrane ultrafiltration in wire-rod tubular modules were derived from momentum and mass balances based on the resistance-in-series model, coupled with the considerations of declinations of flow rate and transmembrane pressure along the membrane tubes. Experimental work for the ultrafiltration of dextran 500 aqueous solution was carried out in a ceramic microporous tube with a twisted wire-rod assembly. Correlation predictions are in better agreement with the experimental results than those predicted in previous works where the flow rate was simply taken with the arithmetic-mean value. The effect of wire angle of inclination on the performance was also discussed. Increasing the wire angle of inclination will increase the fluid velocity in the spiral channel. Actually, the increase of fluid velocity has two conflict effects on permeation: one is decreasing the concentration polarisation which is good for ultrafiltration, while the other is reducing the transmembrane pressure which is bad for performance. It was found that the optimal wire angle of inclination for maximum average permeate flux exists between 45 and 50 in the system of present interest.