Characteristics of load transmission mechanisms in automatic control systems are often affected by existence of backlash and friction due to gears, screw threads and bearings in the mechanisms. This paper dealt with the transmission mechanisms of open loop system from pilot valve to load in a valve-controlled hydraulic servo and their characteristics due to backlash were discussed in relation to load, gear train and stiffness of transmission shaft. The results of theoretical analysis were compared with experimental data obtained from frequency response test at the case of gear ratio 1. As the result, it was proved that the experimental data coincide with the theoretical values satisfactorily. Therefore, the theoretical analysis may be used to improve the performance of transmission mechanisms with backlash.