It has been shown for different High Entropy Alloys that these might show some microstructural instabilities during annealing resulting in a multi-phase structure, which occurs especially rapid in the nanocrystalline conditions and after prolonged times in the coarse-grained states. To gain more insights into the corresponding mechanical properties of differently annealed CrMnFeCoNi samples, nanoindentation was used and revealed drastic changes in hardness, and strain-rate sensitivity but also in the Young’s modulus. Besides a strong increase in the Young’s Modulus due to precipitation formation in the nc-conditions by annealing, a significant scatter in the coarsen-grained states was also found. Further tests in single crystalline but also single phase regions showed additionally a strong elastic anisotropy. High temperature nanoindentation testing was performed at room temperature and additionally at elevated temperatures up to 400°C in order to further investigate the thermally activated deformation components in the nanocrystalline and cast plus homogenized coarse-grained states as well. In the latter case a <100>-orientated grain was selected by electron back scatter diffraction for nanoindentation. It was found that hardness decreases more strongly with increasing temperature than Young’s modulus, especially for the coarse-grained state. The modulus of the nanocrystalline state was slightly higher than that of the coarse-grained one. For the coarse-grained sample a strong thermally activated deformation behavior was found up to 100-150 °C, followed by a diminishing thermally activated contribution at higher testing temperatures. For the nanocrystalline state, different temperature dependent deformation mechanisms are proposed. At low temperatures, the governing processes appear to be similar to those in the coarse-grained condition but, with increasing temperature, dislocation-grain boundary interactions likely become more dominant. Finally, at 400°C, decomposition of the nanocrystalline alloy causes a further reduction of thermal activation. This is rationalized by a reduction of the deformation controlling internal length scale by precipitate formation in conjunction with a diffusional contribution.