Excluding species of the subfamily Bambusoideae which are all C₃ species, identification of C₃ and C₄ species of the grass family in Japan was carried out, then geographical distribution of C₃ and C₄ species in Japan with special reference to climatic conditions was investigated. Obtained results are as follows : 1) Since the CO₂ compensation point is closely related to the "Leaf Anatomy" and no exceptional relationships were observed in this study. Tested species were accepted as C₃ or C₄, depending on the tested results, regardless of whether the distinction was made on the basis of CO₂ compensation point, "Leaf Anatomy". We also classified untested species as C₃ or C₄, where possible, on the basis of their taxonomic affinities. 2) The species belonging to the subfamily Oryzoideae, Arundinoideae, Festucoideae are all C₃ species, and to the Eragrostodeae are all C₄ species. Most of the species belonging to the subfamily Panicoideae are C₄ species, but it is noted that the exceptional C₃ species are also contained in the genus Isachne and Coelachne of the tribe Isachneae, the genus Panicum, Sacciolepis and Opismenus of the tribe Paniceae. 3) High correlations exists between geographical distribution of percent C₄ expressed as percentage of C₄ species in the total grass flora and climatic conditions, especially yearly mean temperature conditions. 4) Increase in percent C₄ of the grass family with latitudinal descend in Japan depends chiefly on increase in species of the subfamily Panicoideae and decrease in species of the subfamily Festucoideae. On the other hand, no remarkable changes in species of the subfamily Oryzoideae, Arundinodeae and Eragrostoideae were found. 5) In the subfamily Festucodeae, it was found that perennial species increased and annual species decreased with latitudinal ascend, at the same time, in the subfamily Panicodeae, perennial species decreased and annual species increased with latitudinal ascend. It is most likely that the life-form of plant is related to the ecological distribution of the grass family.