The phase stability of transition-metal monoxides MnO and FeO under ultrahigh pressure, which reaches the range in the Earth's lower mantle, was studied with the first-principles calculations based on density function theory. The plane-wave basis pseudopotential method was used to perform the structure optimization efficiently, and the electron–electron interaction was treated by the generalized gradient approximation (GGA) supplemented by the LDA + U method (LDA is local-density approximation). Two related structures, normal NiAs (nB8) and inverse NiAs (iB8) types, are emphasized. Our results predict that the high-pressure phase of MnO should take the nB8 structure rather than the CsCl (B2) structure and that a metastable nonmagnetic Bl structure can be realized for MnO in the intermediate pressure range. A very unique iB8 structure rather than the nB8 structure is predicted as the high-pressure phase of FeO, although no materials have ever been known to take the iB8 structure. The novel feature of the iB8 FeO is that the system should be a band insulator in the antiferromagnetic state and that the existence of a bandgap leads to special stability of the phase. The larger c/a ratios for both nB8 MnO and iB8 FeO were explained based on our analysis of the cation radius-anion radius ratios versus c/a for series of similar materials.