The study on the migration of carbon in T23/T91 dissimilar metal welds for the boiler was conducted at $600^{circ}C$ with an increase of aging time from 1 to 100 hrs. Following results were obtained. The heavily etched dark band tended to increase from a molten boundary owing to an increase of aging time, which leads to form hard (T91) and soft (T23) zones with different hardness. It was found that carbon was migrated from T23 area to T91 area due to different carbon activities. In addition, soft and hard zones were formed in regional area mainly resulted from carbide dissolutions and precipitates.