The effect of the heat-affected zone (HAZ) at the crack-starter bead on the nil-ductility transition temperature (NDTT) of drop-weight test specimens of pressure vessel steels was investigated. NDTT varied with changes in thermal cycles in the bead welding and increased linearly with the hardness of HAZ. Using fracture toughness of the base metal and HAZ geometry, NDTT was estimated and compared with the observed NDTT. The difference between the estimated NDTT and the observed NDTT was caused by changes in HAZ hardness. The observed NDTT became lower with softening of HAZ.