This paper presents a new microprocessing method that uses a Cl2 fast atom beam (FAB) with stainless steel (SUS304) patterned masks. This new method uses the patterned mask instead of lithographically processed patterned photoresist materials employed in the conventional FAB microprocessing method. We examined the performance of this method by etching GaAs workpieces under various conditions: (1) by setting the distance between the mask surface and flat workpiece surface, L, from 0 µm to 500 µm; (2) by setting the angle between the FAB axis and the flat workpiece surface, θ; to either 30or 50. (3) by etching a workpiece surface that had a 15-µm step and two different surface textures, smooty and undulated; and (4) by doing overlapped etching using a square-patterned mask first and then a circular-patterned mask. The experiments show that the accuracy of reproducing the mask pattern on the etched surface increases with decreasing L. Moreover, the etching rate is almost the same (L100 µm) and decreases slightly at longer distance (L100 µm). The experiments also show that the side walls of the surface are parallel to the FAB axis, even for θ0, indicating that anisotropic etching can be achieved. The experiments for the stepped surface with different surface textures show the surface texture is not affected by the FAB etching. The overlapped etching experiments show that FAB etching is capable of producing overlapped structures. These results demonstrate that this new FAB method can be used in the microproduction of multi-faced, overlapped, three-dimensional microstructures.