In this paper, experiments are carried out in a 4 m rod-plate air gap under positive and negative lightning impulses. High-speed images of leader development process are captured by using a high-speed camera. Based on the experiment results, the relationship between axial velocities of positive and negative leader and leader position are measured. Moreover, the distribution of the number of leader branches and the angle of leader branches are statistically analyzed. The experimental results show that the axial velocity of positive leaders is positively correlated with the voltage amplitude, while the negative-lead axial velocity is not significantly related to the voltage. In addition, the positive and negative axial velocities increase with the distance from the electrode tip. However, the axial velocity of positive leaders tends to be stable within 1.2 m from the end of the electrode. Moreover, two branches account for 91.8% in the leader branching phenomena, and three or four branches are observed in a few cases. Angles of positive branches and negative branches approximate respectively a lognormal distribution with average values of 44.9 ° and 45.3 °.