The relation between mass transfer and now characteristics in the shell side of a hollow-fiber dialyzer has not been investigated sufficiently because of the tangled flow path. In this paper, the effects of velocity distribution of dialysate on mass transfer in the shell side were investigated experimentally. In order to measure the velocity at various radial and axial positions of a dialyzer, cut dialyzers of various lengths were used. The velocity distributions over the section of the cut dialyzers were measured by means of the electro-chemical method. In addition, the concentration distribution at the cross section of the outlet of lumens of the hollow-fibers was measured when the deionized water and aqueous solution of potassium chloride were fed into a dialyzer countercurrently. Potassium chloride was taken up as a model of low molecular weight species in blood. The velocity was not constant over cross sections of the shell side and the concentration was distributed over the cross section of the outlet of the lumens of the hollow-fibers. These results showed that the mass transfer coefficient in the shell side was varied over the dialyzer. It was considered that the mass transfer coefficient at the local radial position should be discussed in order to make the transport process in the shell side clear. The correlation between the Sherwood number and the Reynolds number based on the velocity and the mass transfer coefficient at local radial positions of the shell side was made. The result of the correlation suggested that the transport process in the center region of the shell side was similar to that in a laminar circular pipe flow.