As widely known, diamond has a higher break-down voltage than other semiconductor materials, 30 times than silicon and 3 times than silicon-carbide. Because of its superior property, diamond has a high potential of high voltage power devices. In addition diamond has unique properties, namely transport property of heavily doped material over 10 20 cm -3, high density exciton being stable even at room temperature, negative electron affinity with hydrogen terminated surface, etc. We have fabricated diamond devices using these properties, such as ultraviolet light emitting diodes, a new type of diode with high breakdown voltage and low ON resistance, high current density flow diode using heavily doped layers, and so on. In this key note lecture, we show the recent achievements of these electronic devices and discuss the potential of diamond semiconductor as a high voltage power device.