A variety of metallic powder particles were thermally sprayed onto the mirror polished metallic substrate surface and the effect of both substrate temperature and ambient pressure on the flattening behavior of the particle was systematically investigated. In the flattening behavior of the sprayed particle onto the substrate surface, critical conditions were recognized both in the substrate temperature and ambient pressure. That is, the flattening behavior changed transitionally on that critical temperature and pressure range, respectively. A transition temperature, Tt, and transition pressure, Pt, were defined and introduced, respectively for those critical conditions. The fact that the dependence both of transition temperature and transition pressure on the sprayed particle material had similar tendency indicated that the wetting of the substrate by the molten particles seemed to be domination in the flattening. Three dimensional transition curvature by combining both transition temperature and transition pressure dependence was proposed as a practical and effective controlling principle of the thermal spray process.