Ultrafiltration is essential during hemodiafiltration treatment especially for removal of medium molecular weight solutes. The effect of solute removal by ultrafiltration was analyzed with membrane hydraulic permeability as a parameter in comparison with the effect of diffusive flow. It was found that ultrafiltration significantly contributes to the removal of medium molecular weight solutes together with diffusive flow, in contrast with low molecular weight solutes, which are mainly removed by a diffusive mechanism. Analysis of the back flow of solute due to back filtration indicated that in order to increase solute clearance it is desirable to apply sufficient operating pressure to overcome back filtration. As a guide to this, the relationship between the amount of back filtration and ultrafiltration volume is shown.