Abstract: The spherical polycrystalline diamond microspheres were prepared on the monocrystalline silicon substrate by microwave plasma chemical vapor deposition. By changing the deposition pressure and temperature, the transition of diamond from graphite growth region to nanocrystalline spherical structure, and to diamond growth region with good crystallization was studied. The particle size increases with the pressure and temperature increasing, and microspheres change from sp³ and sp² coexisting phase to pure diamond phase. The formation of microspheres is mainly controlled by the secondary nucleation mechanism within a certain degree of carbon oversaturation, pressure and temperature. The microsphere grows in the orientation of <110> with the pressure and temperature increasing. The formation of diamond microspheres is controlled by (111) face defects with high density twins and stacking faults, which reveals the growth process of diamond microspheres induced by secondary nucleation mechanism and twin stacking fault mechanism in different growth regions of CVD diamond.