A motorized wheeled vehicle includes a support frame, a head tube secured on the support frame, a front fork pivotally mounted on the head tube, a front wheel rotatably mounted on the front fork, a steering tube pivotally mounted on the head tube and secured on the front fork, a speed reduction control switch mounted on the head tube and provided with a plurality of contact points, and a contact board mounted on the steering tube and movable relative to the speed reduction control switch to press or detach the contact points of the speed reduction control switch. Thus, the speed reduction control switch has a multi-stage and micro-adjustment speed reduction function so as to reduce the speed of the motorized wheeled vehicle gradually when the motorized wheeled vehicle is turned.