We have investigated oxidation behaviors of nanocrystal (NC) -FeSi2/Si composites by Rutherford Backscattering Spectrometry (RBS) and computation of diffusion fluxes. The oxidation is controlled by diffusion of the O flux from the surface and the Si flux toward the Matano-Boltzmann (M-B) interface. The time dependence of the location of the M-B interface was determined. We proposed a model for oxidation of the composite that is different from oxidation of Si controlled by O diffusion through SiO2 at the surface.