Physical property and material mechanical performance of nanocrystalline (single crystal, polycrystalline) CoNiCrFeMn alloy can be known well through an in-depth understanding of the micro-evaluation behaviour of micro dislocation, so that it can better be used in defense fields, such as nuclear reactor cladding tubes, aircraft engines, jet turbine blades and others. In this paper we propose to study the correlation between micro-structure evolution and mechanical properties for nanocrystalline CoNiCrFeMn high entropy alloy. The force driven material deformation behaviors and mechanical properties of nanocrystalline alloy and Ni material are studied by using the nanoindentation method, and effects of temperature on the mechanical properties and micro-structure evolution are compared as well. Research results show that the mechanical properties (maximum load, hardness, Young’s modulus and contact stiffness) of single crystal alloy are superior to those of single crystal Ni, which mainly stems from the fact that the single crystal high entropy alloy with a drum-shape structure is produced under loading period, and the slip and expansion of dislocations in the bulge structure are blocked. At a low temperature (5 K), the maximum load, hardness, Young's modulus and contact stiffness of polycrystalline Ni decrease by 28.9%, 20.27%, 32.61% and 36.4% respectively in comparison with those of single crystal Ni. The maximum load, hardness, Young's modulus and contact stiffness of polycrystalline CoNiCrFeMn material decrease by 21.74%, 23.61%, 23.79% and 22.90% respectively with respect to those of single CoNiCrFeMn high entropy alloy. In addition, the mechanical properties of polycrystalline alloy are more sensitive to temperature than those of single crystal high entropy alloy, whose mechanical properties decrease approximately linearly with temperature increasing. For polycrystalline CoNiCrFeMn and Ni material, the grain boundary is not merely the origin region of dislocation breeding, expansion and reproduction, but also the concentration region of defect initiation, crack expansion and failure. Its mechanical properties are weaker than those of single crystal materials due to micro-structure evolution of grain boundaries driven from stress concentration and defects existence.