The objective of this study is to investigate the quenching of a hot copper slab in water-based silica nanofluid and water at saturated conditions under atmospheric pressure. The slab dimensions are ( 20×20×2 mm ) in such a way that the system can be assumed to be thermally Lumped and heats up to 500 ° C and then immersed in water or nanofluid. The temperature history in the center of slab was accurately recorded and Heat transfer rate was calculated. nanoparticles were synthesized by Tetra-ethyl orthosilicate, ethanol and Ammonia solution as reactants. DLS measurement represents nanoparticle distribution in 3 different sizes:18.94, 32.29 and 63.90 nm with 0.01,0.05 and 0.1% vol concentration. Quenching tests with nanosilica or water were experimented.each experiment repeated 3 times after polishing the slab surface, so that the initial slab surface for all quenching tests is a clean surface of the copper. Quenching and boiling curves for every nanofluid case were compared with water. To investigate nanostructured surface changes by the deposition of nanoparticles, 3D images were taken by (AFM) and (FESEM), before and after the experiments. After quenching in nanofluid, with the increase in particle size, the surface roughness, decreases in comparison with quenching in water by 29%, 33% and 43%, respectively. Also contact angle of nanofluid with surface, increases by 18, 23 and 27 degree respectively, versus contact angle of water, which deteriorates surface wettability. results show that as the nanosilica particles sizes increase for 18.94, 32.29 and 63.90 nm, the CHF values are reduced by 25%, 52% and 59%, respectively, and also the heat transfer coefficient are reduced 11%, 24% and 39%, respectively, in comparison with water and quenching time increases. In this study, since the size of the nanoparticles is smaller than the initial surface roughness, by increasing the size of the nanoparticles, the deposition of particles within the surface cavities, reduces surface roughness, surface wettability and active nucleation sites compared to the base fluid. On the other hand, due to poor thermal conductivity of the fouling layer of nanosilica, additional thermal resistance to heat transfer is caused. Thus, changes in surface characteristics such as roughness and wettability of the surface as a dominant mechanism, reduce the Boiling characteristics, such as CHF and Heat transfer coefficient.