Alumina powder is used to improve the heat conduction and physical properties of heat radiation sheets, these properties can be improved by increasing the filling rate of alumina. Spherical alumina powder is being actively developed to improve the filling rate of alumina in sheets. In this study, a high-temperature flame was created by the flame-fusion method using an oxygen burner to produce spherical alumina. We investigated how the chemical composition and particle size of starting materials affect their fusion behavior under flaming conditions during spherical alumina production using the flame-fusion method. Boehmite and aluminum hydroxide, which accompany the endothermic reaction due to the dehydration of the starting materials, showed lower spheroidization ratios than sintered alumina. The spheroidization ratio improved as the particle size of the starting materials decreased.