It is the dataset assocaited with the figures in the paper titled "Initial leader properties during the preliminary breakdown processes of lightning flashes and their associations with initiation positions" which was submitted to the Journal of Geophysical Research: Atmospheres for publication. It can be visited using Matlab. The abstract of the manuscript is as follows. The properties of initial leaders (ILs) of 1056 flashes during two thunderstorm processes in Guangzhou, China, are analyzed. The median values of IL properties are 11.1 ms for duration, 2.2 km for vertical distance, 2.7 km for 3-D distance, 1.9 × 105 m s−1 for vertical speed, 2.4 × 105 m s−1 for 3-D speed, 28° for angle between the IL’s displacement direction and the vertical direction, 1.1 per ms for pulse rate, 179 m for vertical step length and 224 m for 3-D step length. All the IL properties follow lognormal distributions. Strong monotonic relationships of IL duration with speed and pulse rate and pulse rate with speed and step length are revealed. The upward ILs generally initiated at higher altitude feature greater duration and step length but smaller speed and pulse rate than those of the downward ILs, agreeing with the discovery that with increasing initiation altitude, IL duration and step length tend to increase, and IL speed and pulse rate tend to decrease. The ILs initiated at approximately the −20 ℃ isotherm are more vertically orientated on average than those initiated at lower or higher levels. The ILs initiated in the strong kinematic areas generally had a greater speed and pulse rate but smaller duration, distance and step length relative to those initiated in the weak kinematic areas. A special discussion on the possible impacts of the charge distribution patterns and density on the IL properties in different types of thunderstorms or thunderstorm areas is given.