Powder size, which influence on spray particle behavior in super sonic gas flow in and out of nozzle, is one of the most important parameters in cold spray process. In the other hands, it was reported that small particles (smaller than 5ƒÊm in diameter) decelerate significantly close to the substrate by the bow shock in cold spray. In this study, numerical simulation and experiments investigated the effect of the powder size of copper on the cold spray process and the structures and properties of the cold sprayed coatings. In the numerical simulation, it is assumed that gas flow within the nozzle is quasi-one-dimensional isentropic flow of semi-perfect gas. A spray powder of copper is used. Mean diameter of copper powder 1, 5, 10, 15 @and 45ƒÊm are used. Working gas (is nitrogen) pressure and temperature at nozzle-intake are 3MPa and 350 ‹C. With decreasing particle diameter, calculated particle velocity at the nozzle exit increase immediately. So deposition efficiency increase immediately with decreasing particle diameter until 5ƒÊm. Abstract only; no full-text paper available.