Abstract: In this paper, a method for fracture safety assessment and fatigue assessment was introduced to assess the defectsdiscovered in the welds of GIS shell in one in-service ultra high voltage power system. The weld specimen was preparedfrom the same material by the same welding technology as the original shell. The properties of the base material and theweld specimen were experimentally measured, including mechanical property, fatigue crack propagation and fractureproperty. And fracture assessment and fatigue assessment were carried out on two biggest defects in the longitudinal weldsof the shell by means of the assessment method stipulated in the Safety Assessment on In-Service Pressure Vessels withDefects (GB/T 19624-2004). It is shown that these two defects in the GIS shell are safe under the current working condition.Then, the fatigue life of the defect was calculated based on the fatigue crack propagation rate of the weld specimen. And thefatigue life of the defect in the left cylinder of the shell is 6 971 cycles, and that in the right cylinder is 37 915 cycles. Theestimated fatigue life shall be validated in the future practice. It is recommended to pay much attention to monitoring thefatigue propagation state of defects in the process of operation. And in order to carry out safety assessment, nondestructivetesting shall be performed and compared with current defects in the following 5 years. It is concluded that safe operationstate of this GIS shell since its commissioning is accordant with the assessment result, so the GB/T 19624-2004 is applicableto the safety assessment of GIS shell in high voltage power system.