After industrial revolution energy consumption increased sharply, so threat of energy shortages led scientists to find new sources of energy. During past decades, the price of fossil fuels has increased dramatically. Due to increasing and harmful effect of using fossil fuels on the environment, renewable energy, especially solar energy, should be given more attention than what has been given before. The use of concentrated sunlight as a thermal energy source for production of electricity promises to be one of the most viable options to replace fossil fuel power plants. However, the peak efficiencies of existing fossil fuel combined cycle power plants exceed 50% while those of concentrated solar power (CSP) plants are below 20%. Most concentrated solar thermal technologies today use receivers with absorbing surfaces to convert solar energy from its radiative form into thermal energy. Although surface-based receivers are efficient at solar to thermal conversion, the temperature difference leads to significant emissive losses. Alternatively, in a volumetric receiver design, concentrated solar radiation is directly absorbed and more uniformly distributed in the working fluid, which decreases the temperature difference between the collector and the fluid. One of the proposed ways to increase the efficiency of solar collectors with maintaining the simplicity of the system is direct absorption of solar energy by the bulk of fluid. This study at first presents the parabolic concentrator collectors of direct absorption to investigate the efficiency of the system containing deionized water and then the efficiency of the collector containing aluminum oxide nanofluid with a volume fraction of 0.1, 0.2 and 0.3%. With increasing volume fraction at low flow rates between 10-50 (L/hm 2 ), efficiency has an acceptable process but at higher flow rates, efficiency reduces greatly. Collector efficiency containing an aluminum oxide nanofluid with a volume fraction of 0.3% was evaluated at about 65%. Increasing volume fraction of nanoparticle increases the viscosity of nanofluid; wich would increase exponentially for aluminum oxide nanofluid. On the other hand using nanofluid with low volumetric concentrations has little effect on the pumping power of the cycle. As the volume fraction of nanofluid increases, the stability decrease and particles aggregate and deposit more quickly so must be searched for an optimal point. Investigations have shown that with increasing volume fraction more than 0.6% thermal properties of nanofluids did not increase while stability decreased greatly. Finally, for reducing heat losses and increasing the efficiency of the system some guidelines will be outlined. Key words : solar energy, parabolic collector, direct absorption, Al 2 O 3 nanofluid, efficiency