A method for adjusting central processing unit (CPU) frequency according to the CPU utilization rate in a computer, the method includes the following steps. A Basic Input Output System (BIOS) is booted by turning on the computer. A performance monitor in the CPU is started by the BIOS. A timer is turned on by the BIOS. The system management interrupt program is read by the timer during a time period. A number of clock signals and time values is recorded by the performance monitor in two adjacent time periods. The CPU utilization rate is determined by the performance monitor according to number of clock signals, the time values, and a CPU clock speed. A CPU frequency is adjusted by the BIOS according to the CPU utilization rate. The computer's operation system (OS) is booted by the BIOS.