Abstract: A fan speed control system includes a rack and a plurality of servers disposed on the rack. The fan speed control system further includes the management module and the fan module. The management module is coupled to the servers, and the fan module is coupled to the management module for dissipating the heat from the servers. The fan module contains the fan, the speed control module, and the main control module. The speed control module is coupled to the fans. The main control module has a primary communication port. The main module which enters the interrupt sequence receives the control signals sent from the management module through the primary communication port when any one of the main modules enters the interrupt sequence. The control signal is delivered to the speed control module for reading the fan speed.