Multilayer graphene generated by chemical vapor deposition (CVD) method is irradiated by CO2 laser in vacuum with different power densities and irradiation time. Raman spectroscopy is used to clarify the influences of laser power density and irradiation time on the structure modification. At a laser power density of 13 W/cm2, the decrease of D-band and the increase of G-band intensity are due to the reduction of defects. In contrast, the sample irradiated with higher power density shows that the D-band intensity increase dramatically caused by the laser thermal effects. Nanocrystalline graphene is formed as the defects after irradiation at higher power desities. With increasing irradiation time to 120 s, amorphous carbon is formed at the surface of graphene at a lase power density of 58 W/cm2. These results show that proper irradiation of CO2 laser can improve the charactervstics of graphene.