The microstructure and mechanical properties of high strength 9Cr-0.5Mo-1.8WVNb steel have been investigated to identify the effects of W addition into the 9Cr-1MoVNb steel. The W-added Cr-Mo steel (9Cr-0.5Mo-1.8WVNb) revealed better high-temperature tensile strength regardless of tempering temperature, but exhibited somewhat brittle fracture at higher tempering temperature region, in comparison to those of 9Cr-1MoVNb steel. Transmission electron microscopy also revealed that the evolution of polygonized substructures was delayed by the W addition. The EDS analysis on the structural change of the precipitates, especially (Cr,Fe)₂(C,N), indicated that the W addition increased the ratio of Cr/Fe of (Cr,Fe)₂(C,N), resulting in the stabilization of (Cr,Fe)₂(C,N) through the lattice expansion. It is believed that the stabilized (Cr,Fe)₂(C,N) can enhance the pinning effects on both the movement of dislocation and the growth of lath boundaries. Therefore, it is concluded that the enhanced high-temperature strength of the 9Cr-0.5Mo-1.8WVNb steel is mainly attributed to the retardation of dislocation recovery and the stabilization of carbo-nitrides, caused by the W addition.