Abstract: This paper describes an inportant direction in recent devolopment of mechanical Optimal design. The problem is, namely, the Optimization which involves many discrete veriable.An new method of Constrained nonlinear optimization applied to all or some discrete variable has been studied here as well. This method has stood the test of many practical problems of engineering deeign. The results indicates that the new method is superior both in reliability and efficiency, and is evidently superior to most existing techniques abroad. This method has been used successfully to sove several practical design problems of mechanical engineering. The prospect of its application in the field of engineering design is very inspiring.