The influence of implantation dose on the oxidation behavior of AISI-321 stainless steel was investigated. The steel samples were implanted with Al+ ions of energy 40 keV (doses, 0.5 to 3×1017 ions/cm2) and oxidized in air at 650°C for 48 hr. For comparison purposes, samples were also implanted with a dose of 2×1017 ions/cm2 of energy 200 keV and treated in a similar way. Nuclear-reaction analysis (NRA) and Rutherford backscattering spectrometry (RBS) were used in order to investigate the oxidized samples and to determine the oxygen-depth distribution on the implanted samples. The depth distribution of the steel constituents was determined by secondary-ion mass spectroscopy (SIMS), whereas X-ray diffraction (XRD) was applied to the investigation of the formed oxide scales. The composition and the morphology of the modified region were examined by scanning electron microscopy (SEM-EDS). Al-implantation improves the oxidation resistance of the steel and affects the thermal behavior of the material depending on the implantation dose and energy. The influence of the implantation dose and energy are discussed and mechanisms to explain the experimental results are proposed.