As an important type of powder metallurgy products, tungsten carbide-cobalt (WC-Co) cemented carbides are widely applied in numerous industrial fields. Traditional sintering methods suffer from high sintering temperatures, long sintering times, and grain coarsening, which make it difficult to meet the requirements for the preparation of high-strength fine-grained cemented carbides. In this study, the sintering behavior of the sub-micron WC-5Co (with a mass fraction of cobalt of 5%) cemented carbides was investigated using the high-temperature and high-pressure (HTHP) method. The experimental results show that the sintered body can achieve a relative density of 99.4% and a Vickers hardness value of (24.0±0.3) GPa under the treatment conditions of 5.0 GPa and 1 250 ℃, which is significantly better than that of the same composition of cemented carbides prepared by spark plasma sintering (SPS) and vacuum sintering methods. In comparison with the commercial cemented carbides with the same initial powder, the average grain size of WC in the sintered samples under high-temperature and high-pressure conditions is reduced by about 36%, and the Vickers hardness is increased by about 10%. In comparison with traditional sintering methods, the HTHP sintering method effectively reduces the sintering temperature, shortens the sintering time, and enhances sintering efficiency. Moreover, high pressure can effectively inhibit the abnormal growth of the WC grains during solid-state sintering. During liquid-phase sintering (at 1 450 ℃), an increase in sintering pressure results in significant refinement of the WC grains. When the sintering pressure is increased from 3.0 GPa to 5.0 GPa, the Vickers hardness of the samples is increased by about 7%.