Scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDXS) and in situ X-ray diffraction techniques were carried out to observe the oxide scale evolutions of yttrium implanted and unimplanted commercial 304L stainless steels during and after their high temperature oxidation at 1000degreesC for 100 h. Our results clearly demonstrate that yttrium implantation promotes a faster oxide scale growth and the formation of a more uniform chromia layer due to a higher chromium selective oxidation compared to unimplanted 304L stainless steel. Moreover, the presence of yttrium also leads to the formation of an enriched silicon layer at the metal-oxide interface limiting the growth of iron-based oxides which were not detected (even during cooling) in the case of yttrium implanted samples. These results allow to understand the low weight gain of yttrium implanted 304L stainless steel observed by thermogravimetry and underline the beneficial effect of yttrium implantation on the 304L oxidation resistance at high temperature.