Post- and pre-dilution methods equally have advantages and disadvantages, therefore we should choose the more favorable one for clinical use. However, it still remains controversial which technique is better in on-line HDF. In post-dilution, the clearances of small molecular uremic toxins increase as well as low molecular proteins, however the risk of albumin leakage caused by high transmembranous pressure (TMP) also increases. We can avoid the risk of albumin leakage because there are several maneuvers which aid in avoiding an increase of TMP, such as glucose infusion and gradual control of Q<sub>f</sub>. On the other hand, the pre-dilution method is rather safe in the risk of albumin leakage but has an obvious clearance loss of small molecular substances caused by a decreased dialysate flow rate. The influence of microbial contamination and acetate are rather severe in pre-dilution on-line HDF. These lines of evidence suggest that post-dilution on-line HDF is the best choice for treating chronic HD patients.