An apparatus and method for wirelessly transmitting electromagnetic energy are provided. The apparatus includes a power source, a transmission unit, and a measurement unit. The power source supplies a power according to a certain frequency. The transmission unit receives the power to wirelessly transmit the received power through self resonance. The measurement unit measures a phase difference between a voltage and current of the transmission unit. The certain frequency is controlled according to the phase difference. Accordingly, the apparatus and method control only the frequency of the power supply when a resonance frequency is changed by the change of an ambient environment, thus enhancing energy transmission efficiency.