This study is aimed at investigation of the potential effects of various nanoparticles used in certain properties and ratios in the coolant of VVER-1200 power plant on thermal hydraulic performance. Criticality and thermal hydraulic analysis have been performed by using MCNP and COBRA-IV/PC code, respectively. The fuel ratio has been selected as 90% ThO2 + 10% reactor grade (RG)-PuO2. Al, Cu, Si, and Ag nanoparticles in proportions of 0.01% and 0.1% by volume have been mixed with reactor coolant water. The effects of adding nanoparticles into the coolant at two different ratios on the reactor criticality, departure from nucleate boiling ratio (DNBR), and thermal hydraulic performance have been investigated. It is observed that by adding nanoparticles into the coolant, the temperature rise along the hottest channel is higher compared to the case when only water is used as the coolant, whereas the criticality and DNBR have shown a decreasing trend. It can be concluded that combination of fuel made up of 90% ThO2 + 10% RG-PuO2 and nanofluid containing 0.1% volumetric ratio of Si nanoparticle could be used in VVER-1200 nuclear power plant and this combination could contribute to augmentation of thermal efficiency of the plant.