In this study, a modified flat panel solar thermal collector was built and thermal efficiency was measured with two heat transfer fluids: distillated water and Al2O3-distillated water based nanofluid at high concentration (3.0%) volume fraction of solid phase. In this work for the first time nanofluid with high nanoparticle concentration has been used thanks to a modified solar thermal collector, based on patent WO2011138752 A1, which consists in bottom and top headers properly shaped in order to reduce sedimentation of clusters of nanoparticles. Thermal efficiency has been measured through an experimental setup, according to EN 12975-2 standard. Experimental results showed that an increase of thermal efficiency up to 11.7% compared to that measured with water has been obtained by using nanofluid. Besides effect of nanofluid on thermal efficiency is greater at high temperatures. (C) 2015 Elsevier Ltd. All rights reserved.