A transmission mechanism for a remote-controlled model simulation helicopter includes a body frame which is orderly provided with a main rotor holder, a main rotor shaft, and a main reduction gear provided with a plurality of washout holes which are evenly distributed about an axis thereof. The body frame is provided with a swashplate module and a servo arrangement at a middle portion thereof. Since the main reduction gear is provided with washout holes and the introduction of internally installed swashplate module together with taking advantage of self structural features of the body frame, it enables the elimination of the commonly used swashplate guide board. The servo devices are provided in a same plane within the body frame and provide a triangular configuration, so that the structure is reasonable and the transmittance of motion is stable.