Alloy 617 and Alloy 230 are solid-solution strengthened nickel based superalloys, which have been considered two of the most promising structural materials for the Very-High- Temperature Reactor (VHTR). In order to have a better understanding of the degradation process of the materials in the VHTR, long-term aging experiments have been carried out to investigate the dynamic process of microstructure evolution at 900 and 1000°C for Alloy 617 and Alloy 230. the microstructural evolution process in different aging periods (up to 3000 hours) was analyzed by Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM) and Electron Backscatter Diffraction (EBSD). A diffusioncontrolled precipitation and coarsening of carbide particles (mainly M23C6 and M6C) for both alloys was observed. the corresponding characteristics of the precipitates, i.e. type, size and coherence, were analyzed. the coarsening rate of the intergranular precipitates in Alloy 617 was found to be much faster compared to Alloy 230's. the inhomogeneous precipitation process in the transverse plane of Alloy 617 was observed, which may be attributed to the alignment of the inclusion particles induced by the hot rolling. Hardness and tensile tests were carried out to investigate the aging impacts on materials' strength. Both alloys obtained increased hardness and strength during early stages of aging and softened after elongated time. the results of mechanical tests were in a good agreement with the microstructure evolution process.