We have determined the phase transition boundary between NaCl-type (B1) and NiAs-type (B8) structures of FeO up to 208GPa and 3800K on the basis of synchrotron X-ray diffraction (XRD) measurements in situ at high pressure and temperature using a laser-heated diamond-anvil cell (DAC). The boundary is located at 200GPa and 3200K with positive Clapeyron slope. These results show that B1 phase of FeO is stable along the whole mantle geotherm, whereas B8 phase is stabilized at the inner core condition. Additionally, FeO coexisted with metallic Fe in the present experiments. We found that hexagonal close-packed (hcp) iron is stable over the entire present experimental conditions. Moreover, the direct chemical analyses of the recovered sample demonstrated that solid iron did not contain any detectable oxygen. While extensive solid solution between Fe and FeO has been speculated above 60–80GPa, the present results strongly suggest that the system Fe–FeO is simple eutectic at least to 200GPa pressure range.