Flange is a common part in engineering, and it is tedious to build the 3D models of flanges repeatedly. Nowadays 3D modeling softwares are widely used in many engineering fields such as mining machinery, transportation and petrochemical. In practical engineering, it is necessary to compute the reliability of a flange. For these reasons a reliability design system of flange was developed with VC and Protoolkit. In this system, we only need to input several parameters to build a flange model and to compute its reliability. Also it is easy to determine the key sizes of a flange with an appointed reliability. The application of the software shown that using the software can improve the work efficiency and decrease the labor intensity of designers.