Abstract: By means of numerically simulating mass transfer of ureaat four different placements (placed horizontally with dialysate entranceup, placed horizontally with dialysate entrance down, placed vertically withblood flow direction up, placed vertically with blood flow direction down)of artificial kidney, we qualitatively investigate the effect of gravity onmass transfer performance of hemodialysis, and acquire 3-D concentrationfields and clearances of urea at all placements. The results indicate thatthe effect of gravity on mass transfer of artificial kidney is notable.Under the gravity effect, the clearance with blood flow direction up (moduleis placed vertically) is the greatest, and the clearance with dialysateentrance up (module is placed horizontally) is the second. The results areof significance in improving the clinical efficiency and optimizing design ofartificial kidney.