The mechanism of boron nitride thin film formation using the nitrogen-argon gas mixture plasma jet which is a new method reported by our previous paper has been studied. The electron temperature, electron density and the magnitude of various ion and radical species in the plasma jet have been measured by the electrostatic probe and the optical spectroscopic methods. The amount of recombination of nitrogen free atoms by three body conversion was estimated using the emission intensity of first positive band system from the nitrogen B state vibrational level of v=11. The emission intensity from vibrational level of v=11 at vicinity of the boron target increases more than one order of magnitude larger than that when the boron target is absent and has a peak value at the gas pressure approximately 5 to 10 Torr. Experimental data were discussed in connection with the optimum discharge condition of BN synthesis in the plasma. It is concluded that the enhanced recombination on boron target and substrate surface supplies large amount of molecular nitrogen ions and vibrationally excited nitrogen molecules in the vicinity of the substrate and thus, the high energy ion rich condition makes it possible to form c-BN film on the substrate surface placed in the plasma jet.