Hydraulic control systems, while inherently nonlinear, under many conditions can be studied by means of a linear approximation. Performance may be predicted by this approximation with reasonable accuracy provided the change in fluid flow during a transient is small compared to the steady-state value of the flow. A valve-motor control system is studied by linear analysis and the frequency-response characteristic obtained. The transfer function of this system is of the third order if the servomotor load is a mass. Expressions for the damping ratio, natural frequency, and sensitivity are obtained.