A new analytical model for predicting the mass transport characteristics in hemodiafiltration was presented, which was consistent with in vitro measurements reported by earlier researchers. In this model, Zydneys' equation (1993), modified Rautenbachs' equation (1989) and Fukudas' equation (1992) were used for convective solute transport, and for boundary-layer solute transport on the blood side and the dialysate side, respectively, and the Poiseuille equation was added as a gravity effect term because of vertical setting of the hemodiafilter. The model was applied to study the influence of hollow fiber membrane geometry on the performance of hemodiafilter, and it was found that the clearance of low molecular weight solute depends on fiber geometry more strongly than that of medium molecular weight solute. That is, for a given membrane surface area, the smaller the diameter of the hollow fiber is, the longer is its length and the faster is the blood flow velocity, resulting in larger clearance of low molecular weight solute. On the other hand, the clearance of medium molecular weight solute depend hardly on fiber geometry, but greatly on ultrafiltration flow rate.