Abstract: Diamond thin films were synthesized by plasma assisted chemical vapor deposition in the lab-built 10 kW microwave reactor.The influence of the microwave power on the microstructures of diamond coatings were investigated with scanning electron microscope, Raman spectroscopy and plasma optical emission spectroscopy.The results show that the microwave power has a major impact.For instance, as the power increased, the quality of diamond coating changed in an improvement-deterioration manner accompanied by increasing grain-size, because of larger area of strong field and because of density increases of electron, active H-atoms and C-containing radicals.Deposited at 15 kPa and with H₂/CH₄=200 mL/min, a power increase in 4~5 kW range significantly improved the diamond film quality, and the optimized power was estimated to be 5 kW;however, above 5.5 kW, an increase of power resulted in quality deterioration of diamond film, possibly due to twin-crystal formation.