To provide iron alloy parts improved in surface strength.

In iron alloy parts 1 using iron contg., as chemical components, by weight, 0.1 to 0.5% C, 0.1 to 0.5% Si and 0.3 to 1.5% Cr as a base material, the structure in the range of at least 100 μm from the surface is composed of crystal grains contg. martensite 2 or retained austenite as well and the ones obtd. by precipitating the carbides 3 of Fe or Cr or both or the nitrides of Fe or Cr as well on the crystal grain boundaries by surface hardening treatment (carburizing or carbo-nitriding), the crystal grain size of the martensite 2 or further the retained austenite is controlled to 7 to 10 in the case of being measured and calculated by a grain size measuring method by a microscope in the objective surface for grain size measurement prescribed in an austenite crystal grain size testing method (JIS G 0551) for steel, an average grain size number calculating method based on the same microscopic measurement and a method same as the grain size calculating method, and also, the hardness in the range of at least 100 μm from the surface is controlled at least to ≥700 Hv.