A transmission system for a work vehicle, which is capable of reliably mitigating, with relatively simple arrangement, shocks caused by speed changes even in a work vehicle such as a bulldozer in which almost all operations are performed with the engine operating at full load. In the transmission system which converts the output torque of an engine into torque according to work load by operations for shifting speed gears through disengagement/engagement of friction clutches, if a downshift from a higher speed gear to a lower speed gear is effected while the engine operating at full load, the friction clutch controlling valves, controller and friction clutches allows the friction clutch corresponding to the higher speed gear to slide so as to enable power transmission and the engine controlling fuel injection system and controller allows the engine to perform high power operation (overload operation) during the sliding of the friction clutch.