The diafiltration of a hollow fiber artificial kidney is simulated with a one-dimensional model. The model equations are solved to give an explicit formula for calculating the clearance increase due to ultrafiltration. The computed results are compared with the experimental data of Kunitomo et al. （1977）. The simultaneous diffusion and convection processes occur as if they occur consecutively when the overall permeability is small. The relationship between clearance and ultrafiltration rate is not linear as previous investigators reported because the outlet concentrations are dependent on the ultrafiltration rate.