The effect of oxidation and oxide film on mechanical properties and fracture mechanisms of Si nanostructure is an important factor for the design and application of micro- and nano-scale devices but still remain several debates. Reactive force field molecular dynamics (ReaxFF MD) simulation provides a practical way to investigate such an issue, by which we focused on the role of initial stage oxidation on crack propagation of Si nanofilm. The fracture strain and fracture stress were increased by simultaneous oxidation during uniaxial tension, displaying that the crack propagation retarded and initiated from Si/SiOx interface. The modification of stress field due to oxidation may play as a main part on related strengthening behavior, i.e. the blocking of compression in oxide film and the tension relaxation at crack tip. Moreover, we found that the crack retardation may be associate to oxide film morphology, which indicating further retardation by a high quality oxide film with smoother interface. The mechanism detailed here explained previous simulation and experiment on oxide driven strength evolution of Si surface and may shed a light on strength design of nanofilm with oxide layers.