The present invention enables a decrease in etching velocity in an ion-beam etching system without decreasing the current density of the ion beam. The ion-etching is performed at a constant value of ion-beam current density and an increased partial pressure of oxygen or other suitable gas in the specimen chamber. The partial pressure of oxygen is increased until the intensity of a secondary ion-beam from the specimen reaches a saturation value. Under these conditions a stable reduced etching velocity is obtained, the value of which is determined by the ion-beam current density, the partial pressure of oxygen and the previously determined characteristics of the specimen.