A power transmission system of a hydraulically driven working vehicle comprises: a prime mover supported by a vehicle frame, the prime mover including a prime mover output shaft projecting in the fore-and-aft direction of the vehicle; a pump housing; a hydraulic pump disposed in the pump housing, the hydraulic pump including a pump shaft projecting from the pump housing in the fore-and-aft direction of the vehicle so as to be drivingly connected to the prime mover output shaft; a first hydraulic motor disposed outside the pump housing so as to be fluidly connected to the hydraulic pump; a transaxle supported by one of front and rear portions of the vehicle frame, the transaxle including a transaxle housing, a pair of axles disposed in the transaxle housing so as to be driven by the hydraulic motor, and a differential gear unit disposed in the transaxle housing so as to be drivingly interposed between the hydraulic motor and the pair of axles; a PTO shaft; and a working power train extracting a part of power transmitted from the prime mover output shaft to the pump shaft and transmitting the extracted power to the PTO shaft.