Abstract: An extremely small sized round nozzle is used in air jet impingement experiment, from which the effects of Reynolds number and nozzle-to-plate spacing are investigated in detail. The data of stagnation are well correlated with an empirical formula. Also, the local heat transfer.coefficients obtained from the experiment show monotonously decreasing with increasing of radial distance from stagnation point. An interesting phenomenon is observed from experimental results that flow boundary layer transition from laminar to turbulence will appear with definite Reynolds number and nozzle-to-plate spacing.