Abstract: Mass transfer characteristics of hemodialysis in a hollow fiber dialyzer was studied and a relatively simplified theoretical model was proposed by assuming 2-dimensional and axi-symmetrical flow of a Newtonian fluid accompanied by convective mass transfer across the semi-permeable membrane. The permeation rate was assumed to be small and constant, and the dialysate flow channel was regarded as a perfect mixing-pool. Experiments were carried out by using dog blood in the conditions based on the authers model. Comparison of the theoretical results and experimental date showed that the dependence of mass transfer coefficient on ultra-filtration could be expressed by a linear equation. The error between the theoretical results and expe rmental data was less than 15%.