In this paper, as a basic study of the plasma jet process under a low pressure, the nitriding of titanium plates and atmospheric plasma sprayed titanium coatings are carried out using nitrogen and hydrogen added nitrogen plasma jets at 30Pa. The plasma torch used in this paper is equipped with a supersonic expansion nozzle to improve the acceleration efficiency of plasma jets. The effects of ambient pressures and supersonic expansion nozzles on transformation of plasma particles are examined from gas-dynamical viewpoints. It was observed that from the results, the supersonic plasma jet process under a low pressure was proved to have a high potential for nitriding. Paper includes a German-language abstract.