Mechanical system simulation software is of fundamental importance to computer aided design in mechanical engineering. Such software systems considerably reduce turn around time and have applications in various industries like automative, aerospace and general machinery. A number of mechanical simulation systems are commercially available. However, many of these systems are legacy systems, not necessarily with modern graphical user interface techniques, and also not necessarily designed with newer simulation algorithms or robustness considerations. In this paper we describe the design of a interactive mechanical system simulation package – Clodion. The package has been designed using object oriented design, with careful use of software structuring and also user interface design principles. The design of Clodion has a number of innovative features in the form of its class hierarchy specifically designed for mechanical systems, its event propagation design methodology and its visualization and other utilities. We also present some results which show the user interaction scheme as well.