Abstract: Leader discharge is the main breakdown mechanism in long air gap. A leader channel is a highly ionized plasma channel dominated by thermal-ionization process. Obtaining the temperature distribution of unstable leader channel has great significance for revealing the physical mechanism of leader inception and whole process modeling of leader discharge. In this study, a quantitative schlieren system driven by high power LED light source is established. The quantitative schlieren observation experiments on positive leader discharge with a 1.0 m rod-plate gap are conducted. Throughout the continuous injection of unstable leader current, the axial temperature distribution exhibits relative uniformity, while the thermal diameter demonstrates continuous and rapid radial expansion. Ultimately, the strong convection loss leads to the termination of unstable leader discharge. Besides, it is found that maintaining the gas temperature in the center of channel at approximately 5 000 K is necessary for sustaining the continuous development of leader discharge. It is revealed that heat conduction is the dominant mechanism for energy dissipation during relaxation phase, and the temperature at channel center exhibits a downward trend, eventually stabilizing at 1 600 K, which surpasses the stability value during dark period. This research has important scientific significance and engineering application value for revealing the leader discharge mechanism and supporting the establishment of the leader development model.