The diamond-to-graphite transformation at diamond-stable conditions is studied by temperature gradient method (TGM) under high pressure and high temperature (HPHT), although it is unreasonable from the view of thermodynamic considerations. It is found that, at diamond-stable conditions, for example, at 5.5 GPa and 1550 K, with fine diamond grits as carbon source and NiMnCo alloy as metal solvent assisted, not only large diamond crystals, but metastable regrown graphite crystals would be grown by layer growth mechanism, and the abundance of carbon source in the higher temperature region is indispensable for the presence of metastable regrown graphite crystals. From this transformation, it is concluded that, with metal solvent assisted, although the mechanism of crystal growth could be understood by the macro-mechanism of solubility difference between diamond and graphite in metal solvents, from the point of micro-mechanism, the minimum growth units for diamond or graphite crystals should be at atomic level and unrelated to the kinds of carbon source (diamond or graphite), which could be accumulated free-selectively on the graphite with sp2π or diamond crystals with sp3 bond structure. The diamond-to-graphite transformation at diamond-stable conditions is studied by temperature gradient method (TGM) under high pressure and high temperature (HPHT), although it is unreasonable from the view of thermodynamic considerations. It is found that, at diamond- stable conditions, for example, at 5.5 GPa and 1550 K, with fine diamond grits as carbon source and niMnCo alloy as metal solvent assisted, not only large diamond crystals, but metastable regrown graphite crystals would be grown by layer growth mechanism, and abundance from this transformation, it is concluded that, with metal solvent assisted, although the mechanism of crystal growth could be be appreciated by the macro-mechanism of solubility difference between diamond and graphite in metal solvents, from the point of micro-mechanism, the minimum growth units for diamond or graphite flux shoul d be at atomic level and unrelated to the kinds of carbon source (diamond or graphite), which could be accumulated free-selectively on the graphite with sp2π or diamond crystals with sp3 bond structure.