A new procedure for deriving nonlinear mathematical modeling for a specific class of aerospace hydro - mechanical control valves is presented. The effects of friction on the dynamic behavior of these types of valves along with the experimental verifictions are also given. The modeling approach is based on the combination of the following three tasks: decomposition of the valve into simple specific subsystems; derivation of the governing equations of each subsystem; and determination of the unknown parameters and coefficients using dynamic and static tests. Dynamic analysis shows that the presence of friction causes the hysteresis phenomenon in these valves and friction force increment causes an increase in the tracking error. On the other hand, excessive reduction of the friction force causes an instable performance. Therefore, a trade-off between the amount of tracking error and stability margins must be considered.