Abstract: A control method of a transmission system in a human-powered vehicle based on a forecasting comfortable power consumption and a comfortable cadence is provided in this invention to supply a comfortable travel for a controller of a human-powered vehicle. This control method includes the steps of sensing a traveling distance in one past period and an inclination. Then the controller may forecast a comfortable power consumption in the virtual period and calculate a forecasting traveling distance in the virtual period from the forecasting power consumption. Afterward, controller may calculate a forecasting optimum gear ratio from the forecasting traveling distance and a comfortable cadence and may generate control signals to the transmission system in the human-powered vehicle. The transmission system in the human-powered vehicle then automatically adjust its gear ratio to the forecasting optimum gear ratio or automatically choose an existing gear ratio which is most close to the forecasting optimum gear ratio.