A power management device for multiprocessor systems and method thereof applied to force individual processor entering or leaving of a C3 state are disclosed. The device includes at least one checking unit, a plurality of recording units and a plurality of arbiters. The checking unit receives an event from a peripheral device, checks which processor the event corresponds and sends a checking signal. The event is received and recorded by one of the recording units according to the checking signal. Once the recording unit has no record of the received event, the corresponding processor turns the corresponding arbiter off and sends an entering C3 state command. A first control signal is sent to the processor according to the entering C3 state command so as to force the processor into the C3 state.