Effects of thermal aging at 650 ºC up to 5000 h on the microstructure evolution and tensile properties of 16Cr-3Al ODS steel were investigated. Laves phase and σ phase were identified after thermal aging for 250 h. The Laves phase tended to form at the grain boundaries and exhibited different morphologies at fine grains’ boundaries and coarse grains’ boundaries, while the σ phase mainly located at fine grain zones. The grain structure and misorientation evolution during the aging process were observed by the electron back-scattered diffraction (EBSD) method. It was found that the area fraction of coarse grains increased while the geometrically necessary dislocation (GND) density decreased during thermal aging process, which induced the misorientation evolution. Grain growth mode was discussed by combination with transmission electron microscopy (TEM) analysis. In the zones with uniform grain size distribution, the growth direction of grain boundaries was from the grains with a high number density of precipitates towards those with a low number density of precipitates. Room temperature tensile tests were conducted, and the results of tensile strength, yield strength, and elongation during different stages of thermal aging indicated that 16Cr-3Al ODS steel had a relatively good thermal aging stability at 650 ºC.