The theory of reliability sensitivity analysis and reliability-based optimization robust design are extended to discuss the reliability design of electronic system. Firstly, a method of measuring the relative importance (sensitivity analysis) of each element of system is studied. And then, after establishing the mathematic model for reliability optimization, a procedure for robust design is examined. Finally, a numeric example is presended to illustrate the procedure of sensitivity analysis and robust design. After optimization, the mean life of system has increased than before, and the cost has been greatly reduced; on the other hand, the sensitivity of system reliability to the mean life of components also has decreased. The result shows that the proposed methodology is useful for system design and quality improvement.