In this thesis, the initial wear behavior of the surface layer consisting of an oxide film and a compound layer on gun bores is investigated. The surface layer was produced by the gas-nitriding and the post-oxidization to the surface of the 5.56 mm gun bores. The thickness of the surface layer was measured from the specimen taken from the intact gun barrels with an optical microscope. After real firing, the specimens were cut from the barrels. The microscopic cracking and spalling behavior and phase identification of the surface layer were observed by using a SEM and a XRD, respectively. A lot of surface cracks linked together to form the network of the cracks on the surface and the parallel cracks occurred in the interface between the oxide film and the compound layer. When the surface cracks and the parallel cracks join orthogonally, the spalling takes place.