Size of endosperm and number of cell layers of endosperm were measured for 32 common paddy rice sample varieties (including 4 glutinous and 28 non-glutinous varieties), 4 varieties of large-grain strain and 4 varieties of small-grain strain. On the longitudinal diameter, some varietal differences were observed both in the length and the number of cell layers. However, in the longitudinal length of the individual cells, no varietal difference was found. Accordingly, it can be said that the length of the longitudinal diameter and number of cell layers were in proportion, and the relationship was expressed by formula: Y=28.2X+3.8, where, Y is number of cell layers and X is length in mm of the longitudinal diameter. Although varietal difference in the longitudinal diameter was generally small in case of common varieties, but, in details, a slight tendency was found in that the varieties cultivated in southern districts of Japan were longer than those prevailing in northern districts. In the dorsal, ventral and lateral radii, varietal difference was scarecely found. The dorsal radius was longer than that of the ventral one in all varieties, and that the former was superior to the latter in the number of cell layers. The dorsal radius had a relationship with the breadth (sum of the dorsal and ventral radii) of endosperm both in the length and number of cell layers. There was a tendency that varieties cultivated in northern districts of Japan were superior to those cultivated in south Japan both in the breadth and thickness (twofold of the lateral radius). No difference was found in the cellular structure between glutinous and non-glutinous varieties. There is a very fair possibility that the size of endosperm (elongation and enlargement of cells) was affected by the different cultural conditions, especially by temperature in the early ripening period. On the other hand, the number of cell layers (cell division) was fairly invariable, and the characteristics of a variety might remain unchanged even under the condition under which the size of endosperm might changed. Based upon the results of these observations, it was confirmed that the developmental process of endosperm tissue described in the Reports 1, 2 and 3 of this series using a sample variety Yoneshiro might safely be regarded as common with all Japanese paddy rice varieties.