A power transmission system for a working vehicle comprises: a vehicle frame; a prime mover mounted on the vehicle frame; a pair of left and right drive wheels; a pair of left and right transaxles juxtaposed on the vehicle frame, wherein each of the left and right transaxles includes a single axle for individually driving each of the left and right drive wheels in forward and rearward directions; a working device; and a working power clutch unit disposed between the left and right transaxles so as to clutch on/off power transmitted from the prime mover to the working device. The working power clutch unit includes: a driving side rotary mechanism for receiving power from the prime mover; the driving side rotary mechanism having one or more first rotors interlocking with each other; a driven side rotary mechanism for outputting power to the working device, the driven side rotary mechanism having one or more second rotors interlocking with each other; and a hydraulic clutch drivingly interposed between the driving side rotary mechanism and the driven side rotary mechanism, and incorporated in the working power clutch unit.