In order to resolve the challenge that the load's swing angle and swing angle-velocity of overhead crane or gantry crane were hard to be measured directly in engineering practice, the state observer was designed by using trolley's position information, and estimate the load's swing angle and angle-velocity. The difference value between the state observer's output and the trolley-load system's output was introduced and adjusted by the observer's gain vector, then sent to the observer's input terminal. After the observer's poles were placed on complex plane, the system can estimate the swing angle and swing angle-velocity stably and quickly. Simulation study...