At a youthfully dissected volcano, its initial landf orm can be restored correctly. There-fore, quantitative analysis of erosional landf orm is possible. The result may forms a basis of studies of slope development at general slopes. Erosional process of Mt Yotei, a typical strato-volcano, has been studied by morphometric analysis as an example.<br> Connecting contour-lines smoothly over valleys, initial contour-lines can be obtained. The difference between the radius of the circle with the same area encircled by a certain initial contour-line and that of present contour-line at the same height gives average amount of slope recession at the height. Taking the altitude along ordinate and the radius along abscissa, averaged slope plofiles are obtained. Altitudinal changes of the amount of erosion are as follows at the upper slope the amount of erosion is small and nearly constant or slightly increasing with the distance from the head of the slope. At a certain point of the mid-slope, which is a kind of critical point, it increases rapidly. After reaching the maximum, it decreases rapidly and at the foot of the slope, it becomes nearly zero. Such altitudinal change of erosion is well explained by HORTON's equation on surface erosion. By HORTON's equation, the depth of erosion E at a distance 1 from the watershed is represented as follows;<br> E=K•lⁿ sinα/tan^n/2α-E_m<br> where n is index of turbulence. At the south and the east slope, calculated values obtained by putting n=7/10 (superturbulent condition) well coincide with measured values (Fig. 6). At the higher parts of the slope above the critical point, effects of erosional processes by gravi-tational actions such as soil creeps, landslides etc. have been considered.<br> From simplifying HORTON's equation, partial differential equation on slope development is lead, <br> ∂y/∂t=Kx∂y/∂x+D_m<br> It is assumed from Fig. 3 that the starting point of intense channel downcutting (critical point), the point of maximum erosion and the terminating point of erosion migrate toward the head of the slope as erosion proceeds. The assumption is supported by the solution of the equation with the initial condition of an exponential slope similar with Mt Yotei. It is also confirmed by the results of morphometry at the Iwaki and the Rishiri volcanos.