Abstract: PADS is a parametric design system based on geometric reasoning. It adopts an object-oriented data structure to describe both the geometric model and geometric constraints of a product in a uniform way. Rather than by expert system, the geometric reasoning facility of PADS is accomplished by a fast algorithm, with an efficient inference strategy. Based on geometric reasoning, PADS supports effectively dimension-driven parametric modification of a given design. A user friendly interface is established in PADS to ease user's input burden.