We report on the mechanical friction method for a fabrication of a Si nanostructure on a H-passivated Si(100) substrate using an atomic force microscope (AFM) in a contact mode in air. The bare Si surface region exposed by the mechanical friction between a silicon nitride tip and Si surface was fully oxidized by ambient oxygens. The oxide mask patterns could withstand a selective wet etching process for pattern transfer. The width of the oxide layer formed by an AFM tip was about 200 Angstrom. As the etching time and scan rate were decreased, the oxide line shape was improved. This study also showed that there exists a critical tip force in the removal of a H-passivating layer.