The phase stability of FeO and MnO under high pressure was studied with the first-principles calculations. Our results predict that the high-pressure phase of MnO is a metallic normal NiAs (B8) structure, while that of FeO is the inverse B8 structure (iB8). 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. Analysis of x-ray diffraction experiments provides further support to the present theoretical prediction for both FeO and MnO. © 1998 The American Physical Society.