To ensure the quality of mechanical parts it is necessary to fulfill the basic constructional requirements that are related to the form, function, material and manufacturing procedure. Form of mechanical parts is the result of adjusting all of those requirements, and the task of constructor is the development of forms with the aim of finding the best solution. This paper describes the procedure to determine the effect of geometry changes to stress level in cross shaft Cardan joint. The task of the Cardan joint is mechanical transmission of power and motion between shafts that are changing the position of axis in the process of exploitation and are placed by a certain angle. Cross shaft is one of the most important parts of the Cardan joint. In most cases the size and lifetime depend on the Cardan joint from the cross shaft. The critical stress at the cross shaft was calculated by analytic method and tested by numerical simulation. It is shown how small changes in geometry of cross shaft can lead to a significant reduction in critical stress. By the means of iterative correction of form and repetition of the numerical calculation of stress, favorable ration between the geometry of cross shaft and extreme values of stress has been obtained
