Abstract: The main form of electrical discharge in long air rod-rod gaps is broken down by leader mechanism. The leader development in rod-rod gaps under positive switching impulse was investigated by using two high-speed CMOS cameras. The clearer and more particular colorized physical morphology of the leader propagation process than the traditional image converter cameras was recorded and the leader development parameters were also obtained. As a comparative study, an improved simplified model for the rod-rod configuration was performed to simulate the leader development. The numerical simulations of leader development characteristics were in a close agreement with that of experimental measurements. From the experimental measurements and simulated results, it is obtained that the negative upward leader is incepted at the beginning of the final jump stage and develops to the positive downward leader. Moreover, the propagation velocities of both leaders are almost equal and keep gradually increase with the decrease of the distance of the rest gap. The length of the negative upward leader is almost half of the final jump distance.