A control mechanism for a transmission of a vehicle having an internal combustion engine. The transmission includes a torque convertor, a coupling member, a gear-train, and an operation lever having a neutral position and at least three other positions. The torque convertor, the gear-train, and the coupling member are interposed in a system line for transmitting output torque of the engine to a driving wheel of the vehicle. The operation lever is arranged to operate the coupling member to connect the torque convertor to the gear-train by being moved from the neutral position to one of the other positions. The control mechanism includes a coupling-preventing mechanism operatively associated with the coupling member for preventing coupling when a throttle valve of the engine is in an idling position and the operation lever is moved from the neutral position to one of the other positions. The control mechanism also includes a change-over mechanism associated with the coupling-preventing mechanism for selectively changing-over the coupling-preventing mechanism from an operative state preventing coupling to a rest state permitting coupling whereupon the coupling member connects the torque convertor to the gear-train even though the throttle valve is in an idling position.