In a displacement-control mechanism for a vehicle hydrostatic transmission system including a traveling hydrostatic transmission (HST) and a steering hydrostatic transmission (HST), a steering link member is extended from a steering displacement-control device of the steering HST, and connected at an end thereof to the traveling-direction switching rotary member interlocking with a traveling manipulator for setting traveling speed and direction of the vehicle or with a traveling displacement-control device of the traveling HST. The traveling-direction switching rotary member is rotatable so as to be switched between a forward-traveling position and a backward-traveling position depending on whether the traveling manipulator is operated for forward traveling or backward traveling. A steering-setting rotary member is operatively connected to a steering manipulator for setting turning direction and angle of the vehicle so as to be rotatable centered on a pivotal shaft according to manipulation of the steering manipulator. The steering-setting rotary member is formed with a guide slot in which the end of the steering link member is slidably fitted. Both ends of the guide slot are disposed opposite to each other with respect to an axis of the pivotal shaft so as to move in opposite directions to each other during the rotation of the steering-setting rotary member, and the end of the steering link member is selectively disposed at one of the opposite ends of guide slot depending on whether the traveling-direction switching rotary member is disposed at the forward-traveling position or the backward-traveling position.