Various kind of spontaneous and artificial dwarf mutants have been found so far in cultivated rice plants (Oryza sativa L.) It is assumed that dwarfness is caused by the abnormal differentiation and development of the organs by means of the dearrangement in the metabolic processes. As it was thought that dwarf mutants might be suitable for studying the gene action in higher plant, the authors have engaged in the genetical and physiological studies on the dwarf mutants of rice plants. The present paper deals with the results of the genic classification and morphological investigation of each type. Twenty one varieties of dwarf mutant were crossed each other and with linkage testers. From the results of the progeny test, it was confirmed that all of the dwarf mutants used in this experiment were simple recessive to normal type, and these mutants were classified into following 8 types. (Type/No. of varieties included) 1.Daikoku/10 2.Chokei-daikoku/2 3.Waisei-shirasasa/3 4.Tankan-Shirasasa/2 5.Kotate-tamanishiki/1 6.Bunketsu-to/1 7.Kikei-ban-shinriki/1 8.Shinkane-aikoku/1 Growth analysis and the investigation of morphological characteristics of the dwarf rice plants were made in comparison with normal one (Norin No. 18). The length of coleoptile and seedling height of Daikoku, Waisei-shirasasa, Chokei-daikoku and Kotake-tamanishiki are shorter than those of a normal variety. From these facts, it is supposed that the action of the dwarf genes in these varieties exert relatively in early stage of growth. On the other hand, Bunketsu-to and Tankan-shirasasa were about the same as a normal variety, "Norin No. 18", in weight of an embryo, in length of the coleoptile and in the seedling height. Bunketsu-to reduced, however, the elongation of plant height after the time of development of tillers, and Tankan-shirasasa is suppressed in the elongation of basal internode. These results may suggest that the dwarf gene of Bunketsu-to is closely related to the development of tillers, and of Tankan-shirasasa elongation of internode.