Abstract: Aimed to the time variations of machine parts' stress and strength, a time-dependent mechanical design method, in which time-variable and random factors were taken into account, was proposed on the base of the calculation method of reliability incorporating time varying and uncertainty. The proposed method was analyzed using present time as the starting point. Stress and strength were considered to be affected bY random factors. Also, the distributions of stress and strength at any time t were modeled. A solution to the calculation method was given. Although the method is based on the time-dependent reliability model, it can be applied to other fields such as structural engineering.