An effect of Mg content on thermal properties such as latent heat, solidification range, specific heat and thermal conductivity at high temperature in Al–Mg alloys were investigated. In this study, to determine the solidification range of Al–Mg alloys, the values obtained by DSC test and specific heat test were compared with the ones obtained from equilibrium phase diagram. As a result, the specific heat test is considered to best for determining the solidification range. Furthermore, the relation between solid fraction and temperature was estimated by the curve of specific heat which was continuously measured during solidification. Main conclusions are shown below, (1) As the Mg contents increase the latent heat of fusion decreases and the solidification range becomes wide. (2) The solidification range can be accurately obtained from the curve of specific heat. (3) A method for estimating solid fraction of binary alloy has been established, and this method is able to apply to a multi-component system alloy.