A power management method in a portable electronic device includes storing a plurality of pieces of sub-state information indicating setting states of functions from a user interface (UI) of the portable electronic device, constructing a field by combining sectors, each formed with a predetermined number of bits, indicating the plurality of pieces of sub-state information, analyzing the sub-state information of the field, and selecting a clock frequency corresponding to the analyzed field from among clock frequencies pre-set to operate a central processing unit (CPU) of the portable electronic device and applying the selected clock frequency to the CPU. Accordingly, by combining information on sub-states indicating functions currently set in a portable electronic device in order to construct a field and selecting a clock frequency optimal for the field, an unnecessary increase of a CPU clock frequency in multi-tasking can be prevented, and power of a CPU can be efficiently managed.