In order to make desirable surface layers by low pressure plasma spraying (LPPS), optimum operating conditions and plasma torch gun designs must be decided upon for understanding LPPS plasma and powder behavior. As the first step of this approach, LPPS plasmas without powders were measured by the Thomson scattering, the Michelson interferometry and a Pitot tube. These diagnostics revealed that LPPS plasma jets may be treated as supersonic neutral gas flows as the first approximation. In addition, the neutral particle temperature Tn and ion temperature Ti were found to be the same as the electron temperature Te, which is 1 eV at an oblique shock wave heating point and 0.2 eV at the sugsequent cooling point. LPPS plasmas and flows were modeled by a computer Simulation of a supersonic nozzle flow, 2nd yielded reasonable understanding of the thermodynamic and fluid-mechanical conditions.