Abstract: In this paper, the effect of the distribution tabs atthe dialysate inlet and outlet is investigatedqualitatively by means of 3-D flow field numerical simulation. The tabs are designed to be made of porous material. Then the variationof the artificial kidney's clearance is studied with the porous tabs'Darcy permeability K decreasing from +∞ to 0. The results showthat the clearance first increases slightly, then sharply,when arriving at a maximum, the clearance falls sharply and thendecreases steadily. By this method we may find the optimal Darcy permeability ofthe porous tabs, corresponding to which the maximal clearanceis about 4.5% higher than the current artificial kidney's clearanceunder the same conditions. The results are significant to improve the design of artificial kidney.