An electronic device includes a plurality of processor cores, a core controller, and a core setting module. The processor cores are for executing an application program. The core controller monitors a degree of parallelism of the application program; determining, according to utilization of the processor cores, whether frequencies of the processor cores or power-on number of the processor cores should be changed; and selects, according to the degree of parallelism of the application program, at least one of the frequencies of the processor cores and the power-on number of the processor cores to provide a selection result when determining that the frequencies of the processor cores or the power-on number of the processor cores should be changed. The core setting module changes at least one of the frequencies of the processor cores and the power-on number of the processor cores based on the selection result.