High-quality YB, single crystals were prepared by controlling the composition of the atomic ratio, B/Y, of the molten zone to be 7-8. The preferential growth direction was almost normal to the (112) plane of the tetragonal crystal lattice, which had the lowest Vickers hardness. YB, crystals epitaxially grew on the YB, crystals. The growth rates of YB, and YB, crystals were inversely proportional to the difference in the composition between the crystal and the molten zone.