Actuator is the loading element in Structural Testing System. There are four kinds of actuator. Vertical loading actuator is mostly used to exert vertical load. Quasi-static actuator can be used both as vertical and horizontal loading actuators with working frequency bellow 1 Hz. The only different between quasi-static actuator and quasi-dynamic actuator is that the server valve is installed on the body of quasi-dynamic in order to raise the working frequency to as high as 5～10 Hz. The actuator with the fastest response is called dynamic actuator. It has double end equal area design and can be used in almost all applications who needs quick response and accurate performance.