An experimental investigation was carried out to study the single-phase stagnation point jet impingement heat transfer on smooth and micro pin fin structures using water and R134a. The experiments were carried out for a single jet (d(j) = 2.0 mm) impinging on a 2 x 2 mm micro-heater over a wide range of Reynolds numbers. Both an unfinned and a micro structured impingement surfaces were investigated. The micro structures consisted of an array of 64 circular micro pin fins fabricated using MEMS microfabrication. The micro pin fins had diameters of 125 mu m, heights of 230 mu m, and pitches of 250 mu m with an area enhancement of A(total)/A(base) = 2.44. The jet stand-off ratio and area ratio (A(j)/A(base)) were 0.86 and 0.785, respectively. Nusselt numbers were found to increase with increasing Reynolds numbers. Correlations from the literature for impingement zone Nusselt numbers were found to underpredict the experimental results. Significant enhancement of the heat transfer coefficients were observed as a result of the presence of the micro pin fins on the impingement surface. Enhancement factors as high as 3.03 or about 200% increase in the heat transfer coefficients were demonstrated. Enhancements are attributed to flow mixing, interruption of the boundary layers, and augmentation of turbulent transport. (C) 2011 Elsevier Ltd. All rights reserved.