An overclocking module, a computer system and a method for overclocking are provided. The method is used to overclock the computer system. The overclocking module of the invention includes a timer, a monitoring unit and a control unit. The timer starts to count when the computer system is booted. The monitoring unit monitors whether the computer system performs a boot-up procedure within a period of time. The control unit adjusts an operating frequency of the computer system to overclock the computer system automatically according to the monitoring result of the monitoring unit.