To clarify the relationship between a magnetic property and a defect structure in FeRh inter-metallic compound theoretically, energy band calculations are performed based on the density functional theory. Under the assumption that the majority of defect structure is a type of siteexchanged one between Fe and Rh atoms, total energy for various magnetic structures is evaluated within a super-cell of 2 × 2 × 2 cubic cells. Due to the site-exchange defect pair of nearest neighbor Fe and Rh atoms in 12.5%/f.u. (f.u.: formula unit) density, the total energy increases by 1.91 eV/pair in the anti-ferromagnetic structure and 0.88eV/pair in the ferromagnetic structure. Although the anti-ferromagnetic structure is the stable state at low temperatures in defect-free FeRh, it becomes unstable with an amount of the site-exchange defect density. Threshold defect density to stabilize ferromagnetic state is estimated to be 0.8%/f.u. This phenomenon is expected in ion irradiated FeRh.