Mt. Fuji, the highest peak (3, 776m) in Japan, is one of the most typical strato-volcanoes, and its slopes are slightly dissected by the many radial valleys. Osawa Valley on the westernslope of Mt. Fuji, the largest one among those radial valleys, has attracted the attention of many geographers and geologists, because of its rapid development and violent mass-movements. In this paper, the following two problems are discussed; 1) what are important factors in controlling the formation of various radial valleys on this volcano? and 2) how and since when Osawa Valley has developed? 1) Valleys on Mt. Fuji are divided into the following three types;(A) their heads rise on the upper part of the slope and their tips terminate on the middle part (about 3, 700-2, 600 meters above the sea), (B) their heads start from the middle part of the slope and many of them continue up to the foot of the slope (below about 2, 500 meters above the sea), and (C) their heads start from the foot of the slope (below about 1, 500 meters above the sea). A-type valleys show spoon- or cirque-like forms, and they are estimated to have developed by the erosional agents under the periglacial climatic conditions, i.e. frost actions and snow avalanches. B-type valleys are caused by the actions of impermanent running waters, and are well developed on the western slope, the steepest one of all slopes. C-type valleys distribute almost on all slopes, but more densely on the surfaces constructed by tuff breccias than that constructed by lava flows, and rare on the surfaces covered with new ejectas. 2) The detailed geomorphological and stratigraphical analysis has made clear the following conclusion on the process of development of Osawa Valley. The severe dissection of its velley head was originated from the large scale landslide, which had occurred near the head of ancient B-type valley about 1, 000 years ago. After then, the scar has been eroded by recurrent and so violent mass-movements in the severe climatic condition above the forest limit, that such a large scale valley was rapidly formed. By our estimation, the eroded materials in Osawa Valley amount to 6.5×107m3 during about 1, 000 years and the deposits covering Osawa alluvial fan on the foot amount to 1.5×107m3. Then, about 20% or less of the materials supplied from upper stream of this valley was deposited on this alluvial fan, and the rest might be swept away further downward.