The phase relations of the Fe–Pt–Ho ternary system at 500 °C have been studied by using X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy techniques. The Ho 3 Pt 4 phase is stable at 500 °C, and the introduction of Fe does not cause the Ho 3 Pt 4 phase to be decomposed into the two neighbouring phases HoPt and HoPt 2 . The single phase ranges of α-Fe, Fe 3 Pt, FePt, FePt 3 and Pt in Fe–Pt binary system are from 0 to 10 at.% Pt, 14 to 32 at.% Pt, 33 to 63 at.% Pt, 66 to 78 at.% Pt, and 82 to 100 at.% Pt, respectively. The highest solid solubility of Ho in the α-Fe, Fe 3 Pt, FePt, FePt 3 and (Pt, Fe) phases is less than 1.5 at.% Ho, 2 at.% Ho, 2 at.% Ho, 1.5 at.% Ho and 1.5 at.% Ho, respectively. The isothermal section at 500 °C of the Fe–Pt–Ho ternary alloy phase diagram has been constructed, which consists of 19 single-phase regions, 35 two-phase regions and 17 three-phase regions. No new ternary compounds were found.