Abstract: This paper deals with comparative studies of the root structure in 65 species of 36 genera of Ranunculaceae. Five types of the root structure were recognized in these species. A. The roots, in which the primary xylem was surrounded by the tracheary elements of the secondary xylem and the ray was absent, were always diarchy, as seen in Coptis. B. The rays were narrow and the tracheary elements of the secondary xylem were semicircle in shape, and the roots were diarchy, as seen in Leptopyrum. C. The rays and tracheary elements of the secondary xylem were sector in shape, and the roots were di-, tri-, and tetrarchy, as seen in Cimicifuga, etc. D. There was a few secondary xylem in the roots. D1. Primary structure was diarchy, in Kingdonia. D2. The roots were di-, tri-, tetra- and pentarchy, in Ranunculus etc. E. The secondary structure was absent. El. The roots were diarchy, in Asteropyrum. E2. They were di-, tri-, tetra- and pentarch, as seen in Caltha. According to comparative anatomical studies on the characteristics of the vessel elements and other parts of plant (carpel, petiole and vena- tion), the roots which were always diarcby whether in older or young, and whether the tra- .cheary elements of the secondary xylem well orless developed, could be considered as the most primitive type, for example: type A (Coptis) and type El (Asteropyrum). It would be con- cluded that the evolution of the root structure from primitive to specialized type was in the order from type A→B→C→D2→E2 and E1→D1 in Ranunculaceae.