The present invention relates to a method for measuring a quantity of usage of a CPU, in particular to a method for measuring a quantity of usage of a CPU which is capable of getting a credible quantity of usage of a CPU without amending an algorithm in order to adapt it to the an operating system, e.g., MS-Windows System, or requiring a complicated code. The method uses various algorithms provided by the operating system on the behalf of a registry storing a quantity of usage of a CPU inside a system. Accordingly the present invention can measure a quantity of usage of a CPU easily without lowering a performance of the operating system.