A method and apparatus for designing a screw-tightening element are disclosed for designing the screw-tightening element (mechanical element). The design method comprises representing material characteristics of the screw component with factors including at least one of an elastic limit, an endurance strength, a true breaking power and a tension strength, representing a material characteristic of the tightening object with a factor including an elastic limit, providing a plurality of damage patterns associated with the material characteristics of the screw component and the tightening object, and calculating tolerance values of fluctuating loads in the plurality of damage patterns based on the material characteristics of the screw-tightening element and the elastic limit of the tightening object. The tolerance values of fluctuating loads are compared to determine an optimum tolerance load for the screw-tightening element.