Diamond single crystals are synthesized using artiﬁcial graphite as carbon source and iron-based alloy as catalyst in a cubic anvil apparatus at high temperature and high pressure (HPHT). Four kinds of catalysts at different synthetic times are adopted in synthetic process. After synthesizing the microstructure of the solvent metal samples are characterized by means of scanning electron microscopy (SEM). The results show that when the synthetic quality is relatively superior, the more primary lathy cementites are well distributed and shows parallel growth of the stripe beams. Besides, the edge of the cementite is more even. So the synthesis time and catalyst composition commonly influence the solvent metal microstructure, especially the quantity and shape of cementite.