The microstructure and morphology of nano-granular graphite in situ formed on 50 μm flake micro-graphite irradiated by different energy density laser were investigated. The rules between process parameters of laser irradiation and in situ formation of nano-graphite on micro-graphite were preliminarily discussed. The morphology and microstructure of samples were studied by scanning electron microscope (SEM), and the crystallinity and microstructure of samples were studied by high resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD). The experimental results show that the flake micro-graphite is changed into a large number of spheroidal graphite whose average size is 245 nm when the power density is 5.00 kJ/cm2. When the laser energy density reachs to 6.25 kJ/cm2, the spherical graphite whose average size is 240 nm with agglomerate phenomenon is found. After the laser energy density increases to 12.50 kJ/cm2, samples present as form of spherical particle whose average size is 61.5 nm. When the power density is 13.75 kJ/cm2, the irradiated resultants of flake micro-graphite show a phenomenon that small particles attach to large particles, the size of irradiated resultants have a large range from 150 nm to 280 nm.