Abstract: As a non-invasive plasma diagnostic tool, optical emission spectrum (OES) can detect a variety information of radicals in plasma, and it is possible to analyze this information to reflect the characteristics of plasma, which contribute to investigating the cause and mechanism for single crystal diamond (SCD) depositing. CO₂ is a safer gas than O₂, so there have been more and more studies on introducing CO₂ into source gas for high-quality SCD deposition in recent years. In this paper, homoepitaxial SCDs are cultivated by microwave plasma chemical vapor deposition (MPCVD) with an input power of 4.2 kW. CO₂ is added into CH₄/H₂ plasma, and radicals are diagnosed via OES, and then the effect of CO₂ concentration on the growth quality of SCDs is investigated by combining OES information and Raman spectroscopy characterization. The results show that the increase in CO₂ concentration has an obvious inhibitory effect on the intensity of C₂ and CH radicals, and the strongest inhibitory effect on C₂ especially, which is also the main reason for the decline in growth rate. The ratio of I(CH)/I(H_α) increased slightly, indicating that the increase of CO₂ promoted the deposition of diamond precursors, which weakened the adverse effect on the growth rate to a certain extent. The Raman characterization results show that the quality of SCD grown under the concentration of 0~5% CO₂ improves with the increase of CO₂ concentration, and the impurity peak of 1420 cm⁻¹ basically disappears when the concentration is 5%.