Removal of urea and vitamin B12 was simulated using two pool models with closed loop dialysis, open loop dialysis, and hemodiafiltrations with predilution and postdilulion. It was found that urea removal was limited by dialyzer clearance, however B12 removal was limited both by the resistance between ECF and ICF and by dialyzer clearance. Open loop dialysis was better than closed loop dialysis in terms of dialysate requirement and removal efficiency. Residual renal function plays an important role in removing vitamin B12. Dialysis frequency more than twice/wide does not have great effect on removal efficiency, but has the effect of reducing the difference between maximum and minimum cancentrations during dialysis period.