A sun tracker system, consists of two parts (opto-electronic and hydraulic), has been designed and fabricated to be used in Solar thermal power plant. In this paper various parts of the system including optical sensors, electronic circuits, computational control and mechanical lever have been explained and the operational mechanism of each one is discussed. The parabolic mirror used in this plant has 400 cm length, 570 cm width and 170 cm focal length. Rays falling parallel to the axis of mirror are reflected and collected at the focal point, while unparallel rays are diverted. To determine the rate of divergence, a three – dimensional equation of radiation path is written. Using a computational program in "C" language the error is calculated from "0" to "0.5" degree, for modifying the operational error of the optical system. The optical sensors detect the beam deviation from the mirror,s principal axis with a precision of 0.1 degree and transfer the necessary corrections to the active mechanical system of the hydraulic type. A three phase electromotor of 0.7 kW power and one thousand revolutions per minute controls the mirror movement.