The main functions of excavator are mainly carried out by excavator attachments which are composed of arm and boom. Because the core design factors of excavator such as cost, fuel efficiency and working performance are highly depends on the attachments, the attachments should be designed as light as possible. However the fatigue safety of the attachments should be guaranteed because the excavator is operated under very harsh working environment. In this paper, the virtual design system for the optimum design of the attachments considering light-weight and durability is introduced. The system enables the designer to implement attachments in a relatively short time following automated design procedures. The procedures are composed of mainly 4 parts, which are automated FEM (Finite Element Modeling), stress analysis, fatigue life calculation, optimization for the shape and the thickness. All those parts are integrated to evaluate the initial design and to optimize in view of durability and light-weight.