The purpose of this paper is to make clear the displacement height, and the thickness of roughness region in which the logarithmic equation cannot be applied, as aclue to elucdate the roughness region, and continue investigating the applicability of logarithmic resistance equation in such case. In this paper, we report the results of the experiment on the effect of relative roughness on the velocity distribution in open channel and discuss on the displacement height and the thickness of roughness region with change of constant value of logarithmic resistance equation and turbulent viscosity. A conclusion cannot be drawn due to an insufficient number of experimental samples, it seems to be quite all right to consider that there are effect of relative roughness and river bed slope on the displacement height and the thickness of roughness region.