In this study, the effect of Co addition on the microstructure and creep rupture properties of 9Cr-1.8W-xCo weld metals is investigated. Herein, stick electrodes were fabricated for producing weld metals with 0.5, 1.0, and 1.5 wt pct Co and without Co, which were then processed by post-welding heat treatment at 760 degrees C for 4 hours. The microstructures of weld metals, including the prior austenite grain size, tempered martensite lath size, delta ferrite content, and precipitate quantity and size, were characterized via optical microscopy, scanning and transmission electron microscopy, and electron backscattered diffraction. The precipitates were identified by X-ray diffraction after they were extracted from the matrix. The Curie temperature was determined via differential scanning calorimetry, which allowed the evaluation of diffusion rate as a function of Co content. The weld metals were mechanically characterized by creep rupture tests conducted at 675 degrees C under 150 MPa and by hardness and tensile tests conducted at room temperature as well as Charpy impact tests conducted at various temperatures from - 40 degrees C to 60 degrees C. The results obtained herein suggest that the addition of Co significantly increases the creep rupture time of 9Cr-1.8W steel (by approximately 20 times than that of Co-free steel), reduces the prior austenite grain size and tempered martensite lath size, hinders the formation of detrimental delta ferrite, and promotes the formation of precipitates with slightly larger size. Herein, the highest creep resistance from the viewpoint of creep rupture time was obtained for 9Cr-1.8W weld metal with 1.5 wt pct Co. Furthermore, the results suggest that a finer and more stable substructure is required for improving the creep resistance.