Abstract: In order to study the growth mechanism from graphite to nano-diamond by laser processing, micron graphite suspension was irradiated by nanosecond pulse laser and then was purified by acid high-temperature oxidation. X-ray diffraction, Raman spectroscopy and high-resolution transmission electron microscopy were used in theoretical analysis and experimental verification. The formation mechanism of nano-diamond was analyzed from thermodynamics and kinetics aspects respectively. Ultra-fine nano-diamond with particle size of 4nm~12nm and uniformly dispersion was synthesized. The results show that under the irradiation of nanosecond pulse laser, the transition from graphite to diamond is solid-vapor-liquid-solid phase transformation process. Compared with millisecond pulse laser, nanosecond laser with high intensity and short pulse width can supply larger super-cooling degree for diamond core growth and improve nucleation rate and growth velocity of nano-diamond. However, growth temperature range of nano-diamond is extremely small. Inevitable graphitization on the surface of diamond particles limits the further growth of nano-diamond.