A mechanistic understanding on the aging effects for Alloy 617, which would arise during long-term and high-temperature exposure, becomes very important. The design guideline of mechanical properties due to long-term thermal aging such as tensile and creep properties was not established in current elevated temperature design (ETD) codes. The thermal aging data for Alloy 617 are needed to establish a design guideline, and also the thermal aging effects on creep rupture behavior should be understood well. In this study, for the aged materials which were thermally aged for 1 year at 900oC of Alloy 617, a series of creep tests was conducted with different applied stress levels at 900oC. The creep rupture behavior was comparatively investigated with the virgin material, and the aging effects were quantitatively examined. In addition, the oxide layers and micro-hardness for the aged samples were measured, and the crept microstructures were observed and discussed.