The effect of suction flow on the mass transfer coefficient of tubular ultrafiltration membranes, in particular that under a high-flux condition, was studied. We pointed out that NSh is proportional to (NReNSc0.25)-N-0.875 under turbulent conditions, and that the proportional constant, b, exceeds 0.023 when the effect of suction flow is not negligible. We conducted the velocity variation method using ultrafiltration membranes with MWCOs of 20k and 100k and dextrans having molecular weights of 40,000 and 70,000 at the conditions, where N-Re exceeded 3.6x10(3). We demonstrated that the effect of suction flow includes not only flux but also the diffusion coefficient of solute, and that the ratio of the flux to the diffusion coefficient, expressed as N-Pew, is an important index. Finally, we concluded that b=0.023, when N-Pew is smaller than 2.23x10(3), giving the Deissler equation itself, and that b=2.04310 263N1.21 Pew, when N-Pew exceeds 2.23x10(3). (C) 2017 American Institute of Chemical Engineers