Abstract: In this paper three problems in the nonlinear automatic control systems are analyzed. In the first problem, the monofrequency forced oscillation in the symmetrical non-linear systems is investigated. By using a nomograph (fig. 2) the amplitude AB and thefrequency ΩB. of the forced oscillation at the end of the nonlinear element can be easilyobtained. A stability criterion of the forced oscillation based on the logarithmic charac-teristics is also given. In the second problem, we discuss the effects (error, stability, etc.) of the sinusoidalsignal on the slow changing signal in the symmetrical nonlinear control systems. In the third problem, the amplitude B and the frequency ΩB of the forced oscillationfor smoothing the nonlinear characteristics of the nonlinear element are determined inorder to obtain a system with good dynamic characteristics.