In this paper, heat transfer characteristics of a miniature heat sink cooled by Al2O3–water nanofluids were investigated experimentally. Based on plate fin heat sinks, a new type of plate pinfinnned heat sink is developed which is composed of a plate fin heat sink and columnar pins between the plate fins. The heat sink was fabricated from aluminum and insulated by plexiglass cover plates and consisted of five pin-finned rectangular channels with a length of 42 mm. The volume fraction of the Al2O3–water nanofluid particles was in the range from 0.5 to 2%. Mixtures were prepared without a dispersion agent. Tests were performed while supplying a 180 W/cm2 heat flux to the bottom of heat sink. Experimental results showed that dispersion of Al2O3 nanoparticles in water increased significantly the overall heat transfer coefficient, while the thermal resistance of heat sink decreased. Also, the plate pin-finned heat sink showed an increase in the heat transfer coefficient up to 20% in comparison with the conventional plate fin heat sink.