This study has its for object to plan both utilization and stabilization of slopes in mountain districts by means of doing terracing work on the slopes. Firstly, from the view of hydrological point, the definition of an effect of terracing work was given here, secondly forms of the slopes were classified into three types (one with no terracing work, another with horizontal terracing work and the third with inclined terracing work) and their values of the effects of terracing works were calculatad accord ingly to the definition, mechanism of movement of surface water and discharge of earth and the mea-ning on forestry conservancy and flood control were examined. The followings were made clear. The value of effect of terracing work in the slope with horizontal terracing work is nearly 0 %, on the other hand the one of it in the slope with inclined terracing work is nearly 13 % about the surface water. Thus the latter is clearly larger than the former. Furthermore as for a discharge of earth and sand there is a tendency of errosion's being made more equal relatively in the slope with inclined terracing work than in the slope with horizontal terracing work, judging from a cumulative curve of the discharge ratio of earth and sand.