Abstract: The main stream isolation valve（MSIV） is a key component at the secondary loop of the high temperature gas-cooled reactor（HTR）,and its structural integrity is essential to the safety of the reactor.In this paper,the creep and fatigue characteristics of the MSIV at elevated temperature is studied in accordance with Subsection NH of ASME BPV code Section III,Division 1.Both stresses and deformation of the MSIV subjected to a combination of the elevated temperature,high pressure and seismic load are analyzed,and the damage arising from both creep and fatigue at several critical locations of the MSIV is calculated.The sensitivity of the service life of MSIV to both stress types and levels is also explored.The results demonstrate that high temperature is a key factor that affect the structural integrity of MSIV,and that creep at high temperature dominates high temperature structural damages.