Microstructural investigations of a Fe81Nb9B10 ultrafine-eutectic composite reveal that spherical eutectic colonies are composed of the radial lamellar structure together with NbFeB pro-eutectic phases which are central area on radial structure. The spherical eutectic colonies are proposed to be effectual in high yield strength as well as large plasticity by a rotational motion of those along the colony boundary during deformation. Formation of the wavy and dense shear bands is related with strong interaction between specific microstructure and localized shear deformation. Accordingly, the control of microstructural and mechanical factors in hierarchical ultrafine composite is crucial for enhancement of macroscopic mechanical properties.