In order to study the effect of ion implantation on the in-plane magnetic anisotropy of epitaxial magnetic films, a 3-nm Al buffer layer is epitaxially grown on an Si (111) substrate with a miscut angle, and then 25-nm Fe is grown on the buffer layer. High-resolution X-ray diffraction reveals that the epitaxial Fe film has a (111)-oriented bcc structure. The epitaxial Fe films are implanted by 10 keV N⁺ ions with dose up to 5 × 10¹⁶ ions/cm². The change and mechanism of the in-plane magnetic anisotropy of the epitaxial Fe film are studied systematically. It is found that the in-plane magnetic anisotropy of the epitaxial Fe film is gradually changed from two-fold to six-fold symmetry with the increase of N⁺ implantation dose. It is confirmed by transmission electron microscopy and etching experiments that ion implantation changes the surface and interface state of Fe film. This result is consistent with the result from the SRIM software simulation. The in-plane magnetic uniaxial anisotropy of epitaxial Fe film comes from atomic steps at the surface and the interface of the Fe film. These steps result from Si (111) substrate with a miscut angle. Ion implantation has effects on sputtering and atomic diffusion. The sputtering effect causes the step at the surface of the Fe film to be erased, and the diffusion of the atom leads the step at the interface of the Fe film to disappear. The in-plane uniaxial anisotropy induced by the atomic step is weakened, and the magnetocrystalline anisotropy induced by the Fe (111) plane is dominant. Therefore, the epitaxial Fe film exhibits Fe (111) plane induced six-fold magnetic symmetry after high-dose N⁺ implantation. This work indicates that the in-plane magnetic anisotropy of Fe films epitaxially grown on Si (111) substrate with miscut angle can be modified and precisely controlled by ion implantation. This work may be of practical significance for improving the density of in-plane magnetic recording material.