In structural welded joints after long-term service under elevated temperature, fracture occurred mainly in the heat affected zone (HAZ). Recently, the nucleation and growth of creep voids in the fine-grained HAZ of weldments, recognized as Type IV fracture, has become an important problem for ferritic heat resisting steel. In this paper, a new computational model was presented to analyse the void growth induced creep damage development in HAZ. The new constitutive model based on continuum damage mechanics (CDM) equations is combined with a micromechanism-based model in order to account for the void growth process, which is different from the previous studies of creep damage. Material properties used for the creep damage computations are fitted from actual creep test data. Basic benchmark tests were performed to verify the new computational model. Then the model was used to study the creep damage development in the welded joints where four different material properties, base material, coarse-grained HAZ, fine-grained HAZ, and weld material, are taken into account. The numerical simulation results for creep lifetimes agreed well with the experimental results.