A large diamond crystal up to 500μm in diameter with a smooth(100)facet at its top has been synthesized on Mo substrate through microwave plasma chemical vapor deposition(MPCVD).Its morphology and quality were characterized by scanning electron microscopy(SEM),and the growth mechanism was roughly illustrated from both macroscopic and microscopic viewpoints.It was found that morphological instabilities are a major factor resulting in synthesis of large diamond crystals,moreover,high microwave power density(MPD),high CH4 concentrations,high pressure,high substrate surface temperature and the addition of a small amount of O2 were also necessary for the synthesis of large diamond crystals. A large diamond crystal up to 500 μm in diameter with a smooth (100) facet at its top has been synthesized on Mo substrate through microwave plasma chemical vapor deposition (MPCVD). Its morphology and quality were characterized by scanning electron microscopy (SEM), and the growth mechanism was roughly illustrated from both macroscopic and microscopic views. It was found that morphological instabilities are a major factor resulting in synthesis of large diamond crystals, moreover, high microwave power density (MPD), high CH4 concentrations, high pressure, high substrate surface temperature and the addition of a small amount of O2 were also necessary for the synthesis of large diamond crystals.