A rodless power cylinder includes a cylinder body, a piston assembly adapted to reciprocate in the cylinder body, a magnet mounted at each of the two ends of the piston assembly, a valve mounted at each of the two ends of the cylinder body for guiding a flow of air or fluid in and out of the cylinder body to cause the piston assembly to reciprocate, a magnetic sensor mounted in a sensor mounting groove at each of the two valves for sensing the position of the respective magnet, a movable member mounted around the cylinder body and coupled to the piston assembly by a magnetic force for synchronous reciprocating motion.