Chemical species responsible for diamond growth were examined byinsitumethods using a microwave plasma jet from Ar‐H2‐CH4/C2H2. Irrespective of the reactants, diamond was successfully deposited, and morphology was likely to depend only upon the carbon equivalent concentration in the gas phase. Emission spectroscopy revealed that the C2radical was the main emissive species in the plasma, and that its vibration temperature was estimated to be about 5000 K with little dependence on the axial position. Moreover, emission from just above the substrate showed a considerably lower concentration ratio of H atom to C, including radicals, than the case of low‐pressure microwave plasma. On the other hand, mass spectroscopy mainly detected CH4and C2H2in the species impinging on a substrate. The importance of kinetic processes in a boundary layer was strongly emphasized.