A fan system, which receives an input voltage from exterior, includes a first fan module, a second fan module, a first starting module, a second starting module, a temperature-sensing element and a first controlling module. The first starting module receives the input voltage and starts the first fan module. The second starting module, which is electrically connected to the second fan module, receives the input voltage. The temperature-sensing element produces a sensing signal in accordance with an ambient temperature. The first controlling module controls the second starting module in accordance with the sensing signal so as to start the second fan module.