A method for switching processor performance states, implemented in a computer having a processor and a read-only memory (ROM). The processor includes a frequency field and a voltage field respectively for controlling working frequency and voltage thereof. First, the processor retrieves a frequency identification and a voltage identification from the ROM, bits of which respectively correspond one-to-one to bits of the frequency field and the voltage field, and bit numbers of which are respectively the same as the frequency field and the voltage field. Each bit of the frequency field and the voltage field is respectively filled with a new value corresponding one-to-one to the bit corresponding thereto of the frequency identification or the voltage identification.