Abstract: Diamond films were prepared by Microwave Plasma Chemical Vapor Deposition（MPCVD）.CH₄/H₂/N₂was the main gas instead of CH₄/H₂.Comparison by adding CO₂auxiliary gas and without adding CO₂.We have a study about the effects of different CO₂concentration on the growth of the diamond films.The studies shows：when the flow rate of CO₂was ranging from 0 to 25 cm³/min，surface roughness of the diamond films were 8.9 nm，6.8 nm，9.2nm，9.6 nm.It showed that the amount of CO₂introduced can reduce the surface roughness of the diamond films.Further increase the flow rate of CO₂，the surface roughness of the diamond films even became worse.At the same time，when the flow rate of CO₂was ranging from 0 to 15 cm³/min，the quality and growth rate of the diamond films showed a rising trend.However beyond the critical point.It showed a declining trend.On the other hand，when the flow rate of CO₂was 15 cm³/min，the quality of the diamond films is not only good，but the growth rate of the diamond films is the highest.