For relatively large loads requiredprecise and automatic control, the hydraulic servo system is used. It has many difficulties to determine the cause of failures in the system because electrical controls and mechanical components are interconnected with a complexity. And ensuring reliable hydraulic servo valves to critical equipment is vital to the operation of many industrial, commercial, and military facilities. Despite of the importance of reliability and maintainability, it is impossible to perform actual testing on the system in general. In this paper, the reliability analysis model on the hydraulic servo valve are suggested based on the reliability of individual components which came from fault tree analysis especially if there are no reliability data like past or supplier`s data. The data and methodology presented here are very useful in determining the site reliability or availability.