This paper describes a process simulator that is designed to describe the etching and deposition processes used in constructing compound semiconductors, which have at least two different atomic species. This nature dictates a very different response to compound semiconductor process from the silicon process. One of the most remarkable processes in compound semiconductors is the reverse-mesa formation. This simulator successfully represents the mesa and the reverse mesa profiles that are often observed after chemical etching. The mask material dependence of the undercut etching can also be simulated with a good agreement between the experimental and the simulated shapes.