The phase evolution and magnetic properties of Fe doping into REIn3 (RE = Pr, Nd, Gd, Ho) are systematically studied using X‐ray diffraction, scanning electron microscopy with energy dispersive spectrum, and magnetic measurements. Fe doping into REIn3 induces the magnetic phase transition from paramagnetic to ferromagnetic associated with minor In separation from the cubic phase with AuCu3‐type structure at room temperature. The structural stability of cubic phase is related to size effect RA and electro‐negativity difference ΔX. Cubic REIn3–xFex could be stabilized in the range of 1.0602–1.0963 Å for RA and 0.380–0.420 for ΔX. Ferromagnetism of REIn3–xFex might be explained as the 3d–3d interactions between Fe atoms and 3d–s–4f indirect interactions between Fe and rare earth atoms mediated by the conducting s electrons. The increase of magnetic moment for REIn3–xFex could result from the partial substitution of nonmagnetic element In by magnetic element Fe in the host material through dilute doping of Fe, and thus a net moment is produced.