Objective To study the potential mechanism of the influence of the dialysate flow rate, blood flow rate on hollow fiber dialyzers mass transfer area coefficient. Method We measured KoA for different spatial locations in Syntra 160 dialyzer by selectively sealing the blood-side inlets except one concentric rings where fluid was allowed to enter. The blood flow rate was 120ml/min, the dialysate flow rate was 500、800、1000ml/min respectively. Flow sensitive magnetic resonance images of the dialyzers were used to neasure either blood-side or dialysate-side flow at the different rates Qb and Qd to determine the spatial variation of flow. Result The KoA values in different rings were nearly the same percent in total KoA value when increasing Qd. The MR images demonstrated that the velocity of dialysate increased with increasing Qd but that the distribution of flow remained constant for a given dialyzer. Conclusion The cause of the increase in KoA with increasing Qb may be due to a decrease of the boundary layer at the dialysis side.