The deterioration of creep rupture strength in welded joints of high Cr ferritic heat-resisting steels was investigated based on creep tests of internal pressure specimens and single pass welded specimens. Results showed that at high temperature and low stress cracks occurred in the fine-grained heat-affected zone (FGHAZ) and was identified to be Type IV cracking. It was found that the peak weld temperature between app. Ac1 and app. Ac3 led to the Type IV cracking and many creep voids were observed in the FGHAZ. Further, effects of creep time on specimen necking, void distribution and precipitate coarsening were investigated. It was found that the creep void occurrence and specimen diameter reduction increased at an accelerating rate during creep. Precipitate observation showed that after long time precipitate coarsening was more in the FGHAZ than in the base metal (BM) and in the coarse-grained HAZ (CGHAZ). Auger Electron Spectroscopy (AES) point analysis showed that many precipitates existed in creep voids. Based on this observation, it was suggested that large precipitates were preferential sites for void nucleation. Finally, simulations using a welded joint model and a matrix/precipitate model were performed to investigate creep void occurrence. Results showed that creep deterioration easily occurred in the FGHAZ and large precipitates present in this zone acted as nucleation sites for voids.