The developing velocities of the positive and negative corona streamers on insulating plates have been measured by using the dust figure technique at low pressure in air. By applying nanosecond time pulses from 10‐ to 50‐nsec duration to a needle point where the insulating plate is sandwiched in between the needle and plate electrodes, the diameters of the recorded dincharge pattern against the pulse duration in a pressure range of 150 to 760 Torr were obtained. The maximum velocities of the developing corona streamers which were measured at a voltage of 13.0 kV were 3.0×108cm/sec in the positive case and 2.5×108cm/sec in the negative case for a pressure of 150 Torr. Moreover, when a time pulse of 10 nsec was applied to the needle point at a pressure of less than 250 Torr, the velocity of the positive streamer seemed to reach a limiting value for a given voltage, while the negative increased proportionately to the inverse of the pressure. The resulting polarity effect suggests that when the nanosecond pulse voltage is applied, the field distortion due to the accumulation of the positive ions produced by electron‐collision ionization is one of the most important phenomena for the positive streamer developing at low pressure.