Solar radiation was recognized again as one of the representatives of new energy resources in 1973, when warnings were given to the limitation of fossil resources, and the utilization of solar energy is being investigated very actively in many countries. A large number of studies have been done on the geographical distribution of global solar radiation in Japan. Most of these studies, however, have a tendency to treat the global solar radiation not as energy resources but as one of the climatological or meteorological factors. In this paper, the author newly proposed the Solar radiation-duration curve as the means for arranging the data of solar radiation from the standpoint of effective utilization of solar energy. This is the application to the data of global solar radiation of the Flow-duration curve, which has been used as the means for arranging the data of discharge in the field of hydrology and others. Fig. 2, is an example of the Solar Radiation-duration curve in 1977 at six stations (Choshi, Tokyo, Kofu, Matsumoto, Fukui and Maizuru) situated in the almost same latitude. Here, daily totals of global solar radiation observed at each station during a year were rearranged from large to small. In order to find the indexes to clarify the regional characteristics of global solar radiation in Japan, the author applied the classification of discharge in the Flow-duration curve to the global solar radiation. The classification of global solar radiation is shown on the Table 3. On the basis of this classification, the value of global solar radiation at each station was plotted on the map, and the isopleths at 50 ly intervals were drawn. As a result, the regional characteristics of classified four kinds of global solar radiation in 1976 and 1977 are summarized as follows: 1. The average 95-day solar radiations of all stations in 1976 and 1977 are 412 ly (200J/m2) and 418 ly (202J/m2), respectively. Main isopleths are 350 ly, 400 ly and 450 ly. On the whole, the regions indicating the value over 400 ly distribute from western Japan centering Shikoku, to Tokai and Chubu districts. Conversely, the regions marking the value under 400 ly distribute Kanto district exept for the eastern coastal areas, Hokuriku district, and Tohoku district on the Pacific side. Especially, the extreme low value under 350 ly appears in the inland area of Kanto district (Figs. 3 and 4). 2. The average 185-day solar radiations of all stations in 1976 and 1977 are 269 ly (130J/m2) and 281 ly (136J/m2), respectively. Main isopleths are 250 ly and 300 ly, and the regional difference in the map of 185-day solar radiation is smaller than that of 95-day. It has the same tendency as the distributional pattern of 95-day solar radiation that the regions with the largest value distribute from western Japan centering Shikoku, Tokai and districts. Smaller value appears on the Sea of Japan side of Hokkaido and Tohoku dis tricts, and stretching to Hokuriku and Kanto districts with the value under 250 ly (Figs. 5 and 6). 3. The average 275-day solar radiations of all stations in 1976 and 1977 are 163 ly (79J/m2) and 174 ly (84J/m2), respectively. Main isopleths are 150 ly and 200 ly, and the regional difference in the map of 275-day solar radiation is the same level as that of 185-day. The region with the value under 150 ly appears in the southern part of Kanto district. On the whole, however, the map shows a distinctive distributional pattern that the region of larger value stretchs on the Pacific side and the region of smaller value on the Sea of Japan side. Furthermore, on the Pacific side, the value over 200 ly exists in Shikoku, Chubu and Tokai districts, and the eastern part of Kanto district. On the Sea of Japan side, there is a tendency that smaller value appears in higher latitude (Figs. 7 and 8).