The influences of chromium and chromium oxide films and gas compositions on plasma etching characteristics were investigated. Oxygen as well as chlorine is found to be responsible for etching. One possible etching reaction is proposed, in which a basic reaction product is assumed to be CrO2Cl2 which will be volatile in gas plasma. The impurities such as W, Fe, and Cu contained in the film become nonvolatile compounds which accumulate on the surface of the film and form a masking layer resulting in a suppression of the reaction. Anomalous etching modes called the contour and reverse etches were found with the chromium oxide films containing tungsten impurities. The mechanism of these anomalous etching modes is discussed, in which the accumulation of the tungsten compounds on the surface of the film is a basic mechanism. The reverse etch mode was applied to the fabrication of photomasks for MOS LSI.