Relief valves used in the coal liquefaction equipment are running in an extremely harsh environment with extraordinarily high temperature, high pressure differential, high fluid velocity as well as the intensely slurry flow erosion. The lifetime of relief valves is critically important to the reliability and running cost of the coal liquefaction equipment. Diamond film is potentially used as wear-resistant and protective coating for the relief valve components in coal liquefaction equipment due to its outstanding mechanical and tribological properties. In present study, a layer of high-quality diamond films are deposited on the working surface of conventional WC-Co cemented carbide relief valve components, using the hot filament chemical vapor deposition (HFCVD) technique. Two different types of hot filament arrangements are respectively adopted for depositing diamond films on the interior surface of valve seat and outer surface of valve spool. The fabricated CVD diamond coated relief valve components are characterized using Scanning electron microscope (SEM), X-ray diffraction spectroscopy (XRD), and Raman spectroscopy. The results exhibit that working surfaces of both valve seat and spool are covered by a layer of continuous, homogeneous diamond films on which well-faceted diamond grains with grain size of ~2-3 μm distribute. Furthermore, the fabricated relief valve components are equipped on a coal liquefaction facility and their performance is evaluated in real production. The results present that the working lifetime is largely elongated up to ~1200 hours, as more than three times as that of conventional tungsten carbide valve components.