In this study, we explored the elevated-temperature stability of the medium- and high-entropy alloys for metallurgical processes and applications. Because the extraordinary mechanical properties of medium- and high-entropy alloys are strongly correlated with their distinct microstructural developments, we applied the latest in-situ high-resolution synchrotron X-ray diffraction technique to trace the real-time microstructural evolutions of these alloys during annealing and recrystallization processes. We resolved the convolution information for crystallite size, dislocation characteristics, and planar fault probabilities to examine the microstructural stability and differences between CrFeNi and CoCrFeMnNi alloys. The most significant difference is the grain coarsening of the two alloys subjected to heating from 600 °C to 800 °C. Specifically, the dominant dislocation type of medium- and high-entropy alloys switched from screw to mixed screw-edge after 700 °C. Meanwhile, the screening of dislocation-induced strain-fields of both alloys became stronger during annealing. Our major findings will be useful for the welding and additive manufacturing medium- and high-entropy alloys.