A useful sensitivity analysis of reliability has practical applications in reliability-based design, in optimization of vehicle design, construction, maintenance and inspection under reliability constraints, in parameter studies of the reliability, and in reliability updating. The parametric sensitivity in reliability of the rear axle casing is extensively discussed and a numerical method for reliability sensitivity analysis is presented. The effects of normally distributed design parameters on reliability of the rear axle casing are studied. The method presented in this paper provided the theoretic basis for the reliability design of the rear axle casing.