The very high temperature gas-cooled reactor (VHTR), with dual capacities of highly efficient electricity generation and thermochemical production of hydrogen, is considered as one of the most promising Gen-IV nuclear systems. The primary candidate materials for construction of the intermediate heat exchanger (IHX) for the VHTR are Alloy 617 and Alloy 230. To have a better understanding of the degradation process during high temperature long-term service and provide practical data for engineering design of the IHX, aging experiments were performed on Alloy 617 and Alloy 230 at 900 and 1000°C. Mechanical properties (hardness and tensile strength) and microstructure were analyzed on post-aging samples after different aging periods (up to 3000 h). Both alloys attained increased hardness during the early stages of aging, and dramatically soften after extended aging times. Microstructural analysis including Transmission Electron Microscopy (TEM), Scanning Electron Microscope (SEM), Energy Dispersive Xray spectroscopy (EDS), and Electron Backscatter Diffraction (EBSD) were carried out to investigate the microstructure evolution during aging. A carbide particle precipitation, growth and maturing process was observed for both alloys, which corresponds to the changes of materials' mechanical properties. Few changes in grain boundary character distribution (GBCD) and grain size distribution were observed after aging. In addition, high temperature corrosion studies were performed at 900 and 1000°C for both alloys. Alloy 230 exhibits much better corrosion resistance at elevated temperature compared to Alloy 617.