CAET - I, which stands for Computer Aided Engineering System of Tansha (motorcycle), an efficient design system for frame structure in motorcycle has been developed, incorporating available computer systems for structural analysis and drawing system. As the first step, general methods of dynamic load estimations in motorcycle riding were studied, and computer simulation programs were developed. As the next step, a program was developed to enable the operator to proceed with analysis and design in the conversational mode. Related programs are also available to evaluate fatigue life of stress-concentrated parts such as pipe junctions, gusset corners, etc. All these programs are integrated in the CAET- I system, which easily produces precise initial design in a very short time.