11Cr-0.4Mo-2W-CuVNb steel (ASME Gr.122) has been used for boiler components in ultra-supercritical (USC) thermal power plants. Creep strength of high Cr steel welds decreases due to the formation of Type-IV creep damage in heat-affected zone (HAZ) during long-term use at high temperatures. In the present paper, the creep strength and microstructures of simulated HAZs and a welded joint of Gr.122 steel were investigated. In order to elucidate the processes of Type-IV creep damage, creep tests using thick welded joint specimens were interrupted at several time steps, and the initiation, evolution and distribution of creep voids were measured. It was found that Type-IV creep voids grew after 0.5 of life and a crack formed after 0.9 of life. Creep damage was observed mostly at 1/3 below the surface of the plate. Differences of the Type-IV damage processes between Gr.91 and Gr.122 steel weld were discussed. Experimental creep damage distributions were compared with computed versions using finite element method and damage mechanics analysis.