Abstract In this paper, the Co@SiO 2 core-shell nanoparticles were prepared by the sol-gel method. The oxidization of Co core nanoparticles was studied by the synchrotron radiation-based techniques including in situ X-ray diffraction (XRD) and X-ray absorption fine structure (XAFS) up to 800°C in air and N 2 protection conditions, respectively. It was found that the oxidization of Co cores is undergoing three steps regardless of being in air or in N 2 protection condition. In the first step ranging from room temperature to 200°C, the Co cores were dominated by Co 0 state as well as small amount of Co 2+ ions. When temperature was above 300°C, the interface between Co cores and SiO 2 shells was gradually oxidized into Co 2+ , and the CoO layer was observed. As the temperature increasing to 800°C, the Co cores were oxidized to Co 3 O 4 or Co 3 O 4 /CoO. Nevertheless, the oxidization kinetics of Co cores is different for the Co@SiO 2 in air and N 2 gas conditions. Generally, the O 2 in the air could get through the SiO 2 shells easily onto the Co core surface and induce the oxidization of the Co cores due to the mesoporous nature of the SiO 2 shells. However, in N 2 gas condition, the O atoms can only be from the SiO 2 shells, so the diffusion effect of O atoms in the interface between Co core and SiO 2 shell plays a key role.