Thermal analysis is of great importance when examining the safety and design features of nuclear power reactors. The aim of this study is to examine the effect of pure water and coolant containing Al2O3 nanoparticles on thermal performance of VVER-1000 nuclear reactor. In this study, the VVER-1000 reactor is in steady state and BOC (begining of cycle) condition, relative power density distributions for UO2 fuel with 3.7% enrichment were obtained with the MCNP code and then, thermal analysis were made with the COBRA code and DNBR value, which is very important for the safety criteria of the reactor, was examined. In addition, with the COBRA code, thermal values such as temperature, enthalpy, density, and temperature values of the fuel and clad regions, which are of great importance for the reactor, were calculated. As a result of the analysis, it was determined that the most efficient coolant is 0.1% Al2O3 fluid in the heat transfer of the reactor.