In this paper, the effect of introducting different concentration CO2 into reaction gas on the internal stress of single crystal diamond grown by microwave plasma chemical vapor deposition (MPCVD) homogeneous epitaxy was studied, and its mechanism was analyzed. Results show that with the increase of CO2 concentration, the internal stress of single crystal diamond decreases gradually. This is because the added CO2 provides oxygen containing groups, which can effectively etch the non-diamond carbon in the diamond growth process, and reduce the content of impurities in the diamond, so as to avoid lattice distortion and reduce growth defects. This ultimately shows the reduction of the internal stress of single crystal diamond, which is in the form of compressive stress. Besides, the addition of CO2 in reaction gas can reduce the growth rate and deposition temperature of single crystal diamond, and single crystal diamond with less impurities and high crystallinity can be obtained at a suitable carbon hydrogen oxygen atomic ratio (5∶112∶4).