Microwave plasma-assisted chemical vapor deposition of very smooth diamond coatings is an important process for various applications including mechanical and micromechanical systems and acoustic wave devices. Nanosmooth coatings have been deposited from CH4–CO2 gas mixtures at moderate temperature, the order of 600 °C. In order to increase the knowledge of the process and the control of the final characteristics of the films, a modeling of these plasmas is necessary. This has been carried out here from the prior determination of the plasma parameters. Optical emission spectroscopy was used in order to determine the gas kinetic temperature. Microwave interferometry and Langmuir double probe were used to determine the electron density and the electron temperature, respectively. All these experimental data have been obtained for a wide range of external parameters, such as the inlet composition, the pressure, the gas flow rate, and the power injected in the plasma. Then modeling of CH4–CO2 plasmas was developed by coupling chemical kinetics with a two-dimensional description of hydrodynamics and a surface-wall recombination of main radicals. The kinetic description of the CH4–CO2 plasmas was done by combining a specific mechanism of dissociation by electrons to a slightly modified version of a combustion mechanism for neutral-neutral interactions. This model has been validated by comparing the calculated species concentrations and the experimental results obtained by molecular beam mass spectrometry as a function of various external parameters. The influence of the inlet composition at three microwave power densities has been especially emphasized here. The calculations are in good agreeme- nt with the experimental results. It is shown that among the various parameters that influence the diamond growth from CH4–CO2 plasmas, the power density injected in the plasma is very important as it changes strongly the degree of completion of the chemical system and then the deposition conditions.