This paper presents an integrated computer aided approach to the design of any mechanical device involving modeling, simulation and automatic drawing of the device. The process of computer design is presented on the example of a design of a gear speed reducer using the POWERSIM simulation software. The use of the computer model of a device allows the designer to perform easily the simulation, in order to find optimal technical solution to the elements of the device. In first step the logic of mechanical calculation of a device and their sequence is represented in the form of a flow chart. That helps to develop a computer model in terms of the symbols of a particular simulation software. The next step is a computer simulation which helps to determine the best geometry of the elements of the device. The results of the simulation should be than represented in form of an assembly drawing of the device. Using AutoLlSP language existing in AutoCAD software, the special programs have been developed for automatic drawing of the gear speed reducer. The designer can now observe the final results of his calculations and check the fitness of his decisions taken during the calculation and simulation process. The final step of the development of a particular design is the work of optimization in which the designer is seeking for the optimal solution of the whole device. Any changes introduced at a certain point of the design usually demand certain changes to be introduced to the previous steps of the design. Without computer model of the mechanical structure the process of optimization of the device is difficult and