The electrical breakdown phenomena in liquid nitrogen under atmospheric pressure for point-to-plane electrode configuration have been investigated using a photo optical current measuring technique and a 790 nano-second rectangular high-voltage pulse generator. The waveforms of the prebreakdown current in liquid nitrogen were essentially the same as those in hydrocarbon liquids and in polyethylene. Discharge from positive point was more intense and faster than from negative point. A linear relation to formative time lag and gap spacing was observed for gap length more than some critical values, which indicates an average velocity of the discharge propagation in the most part of gap spacing. The velocities are deduced to be the range 13-18km/s for positive point and 3-12km/s for negative point under the applied voltage range 20-50kV. These values are an order higher than those in hydrocarbon liquids and are same order as those in chlorinated liquids for both point polarities.
