We propose a stacking-disordered phase defined as a thermally equilibrated planar defective hexagonal-closed-packed structure for a stable state of iron under the Earth's inner core condition. A multiple configuration sampling technique based on first-principles calculations shows that a stacking fault concentration reaches approximately 40% at a pressure of 400 GPa and a temperature of 6000 K. In addition, the stacking disordering is found to enhance elastic anisotropy, which is expected to explain seismologically observed P-wave anisotropy at the Earth's inner core. © 2011 American Physical Society.