Erosional processes of youthfully dissected strato-volcanoes in Japan have been studied quantitatively by physical and morphometrical analysis. The result may be applied to the study of slope development at general slope.<br> The amount of detritus eroded by tracting force of overland flow is given by a simplified function of slope length l and the sine of slope angle θ as follows;<br> E=Kl^msinⁿθ (1)<br> where K is a function of climatic, vegetational and geologic factors. K is supposed to be con-stant at each mountain slope assuming the mean condition in the long geological time. The value of m and n obtained by formulas on tractional load proposed by Brown, Shields, Du Boys and so on approximately ranges from 1.0 to 1.7. The amount of detritus eroded by secondary action of mass movement such as landslide and debris flow is given by the same equation. In this case, the value of m is larger than the above value. At actual mountain slopes, values of m and n are supposed to be smaller than theoretical ones, and to vary with slopes related to mainly the sorts of erosional phenomena there occur. The above equation of erosion may be applicable to fairly short and steep slopes.<br> Averaged profiles of initial landforms and altitudinal changes of average amounts of ero-sion were obtained by morphometric measurement at seven slopes of Iwaki, Haruna, Nantai, Takachiho and Yotei volcanoes. Gradients of the lower ends of the measured slopes where the amounts of erosion come to nearly zero are 15-20 degrees. The gradients approximately coincide with the critical angle of slope surface deposited by debris flow. Then, by introduc-ing the critical angle θ_c, equation (1) is modified as follows;<br> E=Kl^m(sinθ-sinθ_c)ⁿ(2)<br> By the method of least squares, multiple regression coefficients, log K, m and n were obtained with each slope. Substituting l, θ and θ_c obtained from the averaged profiles for the equation with determined coefficients, theoretical values of erosion are obtained at any place of each slope, and agree quite well with measured values. As Mt. Iwaki has long and gentle slope, equation (2) is applicable only to the upper slope, but Horton's equation on erosion can be ap-plied with success at the lower slope. Curvature of slope is infered to have only a little in-fluence on erosion.<br> From simplifying equation (2), partial differential equation on slope development is lead, <br> ∂y/∂t=Kx(∂y/∂x+I_c)