We have studied the growth of Fe thin layers on In-terminated InAs(100) surfaces by X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES) and reflection high energy electron diffraction (RHEED). Islanding is observed at the early stages of Fe deposition, then the growing mechanism is layer-by-layer. In thicker films, iron atoms are arranged in an unstrained bcc structure with a lattice constant close to that of hulk alpha-Fe. Arsenic diffuses through the Fe layer within a typical diffusion length of around 7 Angstrom. In turn, indium atoms remain at the interface.