Quenching of small stainless steel rods in pure water and nanofluids with alumina and diamond nanoparticles at low concentrations (0.1 vol%) was investigated experimentally. The rods were heated to an initial temperature of -1000°C and then plunged into the test fluid. The results show that the quenching behavior of the nanofluids is nearly identical to that of pure water. However, due to nanofluids boiling during the quenching process, some nanoparticles deposit on the surface of the rod, which results in much higher quenching rate in subsequent tests with the same rod. It is likely that particle deposition destabilizes the film-boiling vapor film at high temperature, thus causing the quenching process to accelerate, as evident from the values of the quench front speed measured by means of a high-speed camera. The acceleration strongly depends on the nanoparticle material used, i.e., the alumina nanoparticles on the surface significantly improve the quenching, while the diamond nanoparticles do not. The possible mechanisms responsible for the quench front acceleration are discussed. It is found that the traditional concept of conduction-controlled quenching cannot explain the acceleration provided by the nanoparticle layer on the surface.