In this paper, thermal-hydraulic effects of nanofluid as coolant in VVER-1000 nuclear reactor with annular fuel are investigated. At the first, the core of a VVER-1000 reactor is designed based on the use of internally and externally cooled annular fuels and thermal-hydraulic parameters of the fuel rods are analyzed. From the neutronic viewpoint, Alumina is the best nanoparticle for normal operation at low concentration. In this paper, for this nanoparticle, fuel assembly is simulated in the hot channel using CFD (Computational Fluid Dynamics) simulation codes and thermal-hydraulic calculations (maximum fuel temperature, fluid outlet, MDNBR (Minimum Departure from Nucleate Boiling Ratio), etc.) are done. As one of the most important results of the analysis, using the nanoparticles, the heat transfer coefficient in outer coolant, which was already decreased using annular fuel, is increased. Also, by applying the nanoparticle with smaller size and major concentration, MDNBR is increased. (C) 2016 Elsevier Ltd. All rights reserved.