Based on linear quadratic Gaussian (LQG) theory, this paper shows a study of a LQG optimal guidance law that minimizes the terminal miss distance and control energy consumption within the intercepting period. This paper also gives an analysis of the law's effective navigation ratio and these ratio's applications to the tactical missiles. The results obtained by using adjoint methods show that the optimal LQG guidance law has an advantage over the traditional one in control accuracy and maneuver overload.