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The initial radius of the return stroke is one of the important parameters reflecting lightning discharge intensity. Based on spectral diagnosis and electrostatic approximation of the vertically descending leader model, the lightning channel temperature, line charge density, and electrostatic energy of seven dart leader–return stroke sequences were estimated. Finally, the initial radii of the return strokes were obtained. We analyzed the relationship between the initial radii and the corresponding discharge parameters. The initial radius is found to have a good linear correlation with each of the following quantities: the line charge density of the dart leader, the total intensity of ionic lines in the spectra, and the optical radius. It is also inversely proportional to the leader duration. These correlations confirm that the initial radius is closely related to discharge intensity, and that the charge stored in the leader channel directly determines the initial radius and discharge intensity of the return stroke.