Abstract: An optimal guidance law which considers kinetic factors of missile and target via first order inertial loop in three dimensional space is studied on the basis of the theory of differential games. An optimal feedback guidance law is given in vector form, and the control rigidity parameter"k"is introduced into the feedback gain. The "k" is a scalar with determinate physical meaning. It represents responded characteristics and controllability of the system. Hence, in addition to being a function of time the feedback gain is related with the characteristics of the system. So it is possible to develop the certain relations of the feedback gain to other parameters such as Ts, Ks,ξs as well as H, M etc. by further studying on the parameter"k". Therefore, an improvement for the effect of the guidance law can be obtained, In other words, since the control rigidity"k" is introduced, a new synthesis mean is provided to the further studying on the guidance law.Finally, several problems are discussed in brief: first, the degeneration of the optimal guidance law proposed is discussed under certain conditions; then, for the convenience of realization, a suboptimal guidance law is given in finite rigidity case by means of further simplification; and also the controllability of the system is illustrated essentially. In addition, the case of ξ≠1 is considered in Appendix.