Abstract: This article deals with the application of cybernetics in the design of mechanical transmission system. Just as cybernetics is applied to the design of information transmission system, it is applied to the design of mechanical transmission system, so as to substitute the traditional rules of thumb. In the light of cybernetics, the author has analysed the influence of the parameters of the mechanical transmission system of the contouring NC milling machine on its working: accuracy, and the author has taken the actual measurements of the working errors. The theoretical analysis is in conformity with the actual measurements, hence is justified the feasibility of the cybernetics to the design of mechanical transmission system. In this article are given the mathematical models and the transfer functions of the typical mechanical transmission system the analysis of the errors in static and dynamic states; the feedback control systems and the mechanical governors and their mathematical models; transfer functions and the amplitude-frequency, curves in logarithm. A new approach to raise the accuracy of the mechanical transmission system is thus obtained.