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Oil-based nanofluids including 0.75％-1.75％ (mass fraction) carbon nanotubes (CNTs) without any surfactants have been synthesized by a two-step process.The probes machined from 45# steel with 22 mm diameter and 50 mm length are quenched in the as-synthesized CNT nanofluids for testing the cooling behaviors of the nanofluids.The laser diffraction particle size analyzer,scanning electronic microscope (SEM),X-ray diffractometer and transmission electron microscope (TEM) are used to characterize the quality and distribution of CNTs in the nanofiuids.The wettability and viscosity of 30# oil and oil-based CNT nanofluids are measured by a goniometer at 15 ℃ and a rotational type viscometer at 40 ℃,respectively.The results show that the cooling efficiency of the oil-based CNT nanofluids is better than that of 30# oil.Moreover,the cooling rate of the naonoflnids increases with the further increase of the CNT concentration.When the mass fraction of CNTs increases to 1.75％,the cooling rate of the naonofluids reaches a maximum at 760 ℃ and it is increased by 77.8％ as compared to that of base oil.The improved cooling rate of oil by CNTs is mainly due to the uniform distribution and excellent thermal conductivity of CNTs.