Experimental investigation of heat transfer during pool boiling of two nanofluids, i.e. water-Al2O3 and water-Cu has been carried out. Nanoparticles were tested at the concentration of 0.01%, 0.1%, and 1% by weight. The horizontal smooth stainless steel tubes having 10 mm OD and 0.6 mm wall thickness formed the test heater. The experiments have been performed to establish the influence of nanofluids concentration on heat transfer characteristics during boiling at different absolute operating pressure values, i.e. 200 kPa, ca. 100 kPa (atmospheric pressure) and 10 kPa. It was established that independent of nanoparticle materials (Al2O3 and Cu) and their concentration, an increase of operating pressure enhances heat transfer. Generally, independent of operating pressure, sub- and atmospheric pressure, and overpressure, an increase of nanoparticle concentration caused heat transfer augmentation.