The work aims to study law of influence of CO2/CH4 flow ratio on growth of diamond films and grain size. The diamond films exhibiting different structural features were prepared controllably by adjusting the CO2/CH4 flow ratio and applying MPCVD technology. The diamond films were characterized and analyzed with scanning electron microscope, X-ray diffractometer and Raman spectrum, the rule of influence of CO2/CH4 flow ratio on grain size of diamond films was obtained. When microwave power, deposition pressure, substrate temperature and of CH4 flow was 1.2 kW, 7.0 kPa, 850 ℃ and 50 mL/min, respectively, nanocrystalline diamond films could be deposited using 20 and 25 mL/min CO2, microcrystalline diamond films could be obtained using 30 and 35 mL/min CO2, and diamond could be deposited using 67 mL/min CO2. By adjusting microwave power to 0.9, 1.4, 1.8 kW, respectively while keeping the other parameters constant, grain size variation of diamond films along with the CO2/CH4 flow ratio could be divide into three regions: nanocrystalline diamond film deposition region (CO2/CH4<50%), microcrystalline diamond film deposition region (CO2/CH4>60%), and grain size transition region (50%