Abstract: The new national criterion (GB3480-83) for gear design has provided evidence for the Application of CAD in gear design.This paper establishes CAD and intelligent software of generaly used involute cylinder gear reducer on IBM PC/XT, AT Micro-Computer with the help of Expert System and CAD technique.The data tables in the software come into being according to the frame stru cture in Macro-LISP language, the reasoning procession uses with combination of productive rule, predicate logic and frame structure. The design results sto red in data file for the purpose of applying in drawing file.The application of the software GDDES proves it is a successful expert system for gearing design.