The experimantal phenomena for the formation of graphite, -the metastable phase of carbon -from molten metal solutions under ultrahigh pressures and high temperatures are given. Taking this as an example, we have analyzed the thermodynamic possibility of the crystallization of the metastable phases from the solution. The concept of supersaturation that promotes the metastable phases to crystallize from solutions has been presented. It has been pointed out that the temperature difference in a solution system must be the driving force for crystallization of the metastable phase from, the solution. The emphasis is placed on the calculations of the critical temperature difference of the possible crystallization for the metastable graphite under a given crystallizing condition.