The X20Co martensitic heat-resistant steel (X20Co) has notable characteristics such as elevated high-temperature strength, as well as commendable resistance to oxidation and corrosion. The X20Co has been extensively employed as high-temperature working components of die-casting machines for Mg-alloys. Nevertheless, the prolonged exposure of die-casting machine components to high-temperature magnesium alloy liquid might result in deformation and fracture failure due to the occurrence of high-temperature creep. Therefore, it is imperative to enhance the high-temperature creep resistance of X20Co and prolong the operational lifespan of hot-work components in die-casting machinery. Rare earth (RE) elements, including Ce, are seen as potential means to improve the creep properties of X20Co. However, the effect of Ce on the microstructure and creep performance of X20Co have not been reported yet. In this study, the impact of Ce on the creep properties and microstructural characteristics of X20Co by applied stress within 100~200 MPa at 680~720^oC is investigated by means of electronic creep testing machine, scanning electron microscope (SEM), transmission electron microscope (STEM), and energy-dispersive X-ray spectroscopy (EDS). The findings indicate that the incorporation of Ce can enhance the creep resistance of X20Co. Moreover, as a subsequence of the increase of Ce concentration, there is a remarkable and substantial improvement in the creep life of X20Co. As an illustration, by the testing condition of 700^oC/150 MPa, the X20Co steel with 0.005% and 0.012%Ce (mass fraction) presents enhanced creep rupture time c.a. 33% and 103% respectively superior to that of the plain X20Co. The creep stress exponent, activation energy, and threshold stress of the plain X20Co are determined to be 5.05, 572.3 kJ/mol and 58.3 MPa, respectively. Correspondingly, those of the X20Co with 0.005% (mass fraction) Ce are 4.76, 595.0 kJ/mol and 87.8 MPa, respectively. Whereas, those of the X20Co with 0.012% (mass fraction) Ce are 4.49, 642.1 kJ/mol and 82.5 MPa, respectively. It is indicated that the creep processes of the three X20Co steels all follow the mechanism of dislocation climbing. It is a fact that the addition of Ce has not changed the creep mechanism but clearly raised the creep activation energy and threshold stress for X20Co steels. Furthermore, the microstructural evolution analysis compared before creep and after fracture reveals three distinct precipitates appear in X20Co. These precipitates are the large W-rich M₆C phases and Cr-rich M₂₃C₆ phases on grain boundaries, as well as fine V-rich MC phases within grains. It is suggested that Ce reduces the number of large-size massive M₆C phases, which significantly improves the creep properties of X20Co.