During last decade, the appearance of high entropy alloys (HEAs) attracts so many attentions because of their unique phase structures and excellent properties.Very recently, a significant phenomenon was found in CoCrFeMnNi alloys. Generally, the fracture toughness of CoCrFeMnNi HEAs remains unchanged with decreasing temperature, while their strength shows an expected increase. Nevertheless, the underlying mechanisms remain unclear.Therefore, in-situ experimental investigation of the low-temperature deformation mechanism of HEAs is urgently required. In this proposal, we aim to trace the low-temperature deformation behavior of CoCrFeMnNi and HfNbTiZr HEAs in order to disclose the origin of excellent cryogenic mechanical properties in these alloys.