This paper introduces the development of a three-dimensional jig design system for automotive body welding assembly. Recently, three-dimensional CAD systems have been introduced in the jig design area, because they reduce errors in design process and the design result can be used readily for virtual manufacturing simulation. However, to facilitate three-dimensional jig design, it is essential to customize three-dimensional CAD system for this specific design area. To accomplish this object, we first standardized the parts and units in a jig assembly, and then built the standard part library. We also developed the dedicated 3D design capabilities of jig units and a jig base. By using this system, design can be performed more intuitively, and verification and simulation of design results can be done more easily. The proposed system has been implemented using the UG/Open API of the Unigraphics system.