Alloy 617 is considered as a leading material used for next generation nuclear power plant due to its good corrosion resistance and exceptional high-temperature strength. In the present work, the effects of long-term aging (up to 3000 h) at 900°C and 1000°C on the microstructure and associated mechanical properties are investigated. Specimens from the rolling plane and transverse plane were selected to study the anisotropic effect, which was originally induced by carbide inclusions from the hot rolling processing. Microstructure characterization by transmission electron microscope (TEM), scanning electron microscope (SEM) and electron backscatter diffraction (EBSD) revealed the precipitation evolution processes for different aging conditions. Precipitate type, size, location and coherency were investigated. Hardness and tensile tests showed that the influenec of the in-plane anisotropy was significant during the aging process, which may be attributed to diffusion-controlled coarsening of the precipitates.