The robustness of ESD (electrostatic discharge) is decreased by increasing frequency and data transfer rate. An ESD and EMI (electronic magneto interference) problem arises when measures for reducing ESD and EMI do not meet the immunity level of the device. In order to accomplish proper countermeasures against ESD and EMI, it is necessary to investigate the phenomena. An electromagnetic wave caused by the ESD has a broad-band and high energy which is concerning factors for electronic devices such as semiconductor, MEMS and Magnetic head. In this paper, we investigated the characteristic of radiated electro-magnetic wave in a HDD enclosure by ESD injection with contact or air gap discharge. It was found that the phenomena of enhanced radiated electromagnetic wave in low voltage by air gap discharge due to lower discharge resistance and higher rise time.