An overclocking method applied to a computer system includes the following steps: setting a first operating voltage and a first clock rate; generating a first control signal to a power supply and generating a second control signal to a clock generator according to the first operating voltage and the first clock rate, respectively; controlling the computer system into a sleep mode; resuming the computer system from the sleep mode after a predetermined time; restarting the power supply and the clock generator, and generating the first operating voltage by the power supply according to the first control signal and, generating the first clock rate by the clock generator according to the second control signal; and setting a parameter of a memory controller in a north bridge chip of the computer system via the first clock rate and the first operating voltage.