The Yb2O3-Fe2O3 system was studied to investigate the effect of oxygen partial pressure on the formation of metastable phases over a wide range of oxygen partial pressures from 10(5) to 10(-1) Pa. Two kinds of metastable phases, with space groups of P6(3)cm and P6(3)/mmc, were found through rapid solidification of an undercooled YbFeO3 melt in an atmosphere with reduced Po-2. The crystal structure of the as-solidified samples changed from orthorhombic Pbnm to hexagonal P6(3)cm and P6(3)/mmc with decreasing Po-2. X-ray diffractometric and scanning electron microscopic results confirmed the existence of various phases in the as-solidified samples. The stabilities of each phase were studied by annealing the bulk sample in the thermogravimetric-differential thermal analysis (TG-DTA) furnace up to 1673 K, and the equilibrium phase diagram was constructed for the Yb-Fe-O system at 1473 K. TG analysis showed an increase of the sample mass during annealing and revealed that the existence of Fe2+, which has an ionic radius larger than that of Fe3+, decreases the tolerance factor and therefore destabilizes the perovskite structure.