Deformation mechanisms of high entropy alloys (HEAs) at cryogenic temperatures have attracted extensive research interest. We used in situ neutron diffraction to study the tensile behavior of a face-centered-cubic HEA at 77 and 15 K and compared its stacking fault energy (SFE) at ambient and cryogenic temperatures. The SFE dropped from 28 mJm-2 at 293 K to 11 mJm-2 at 15 K, leading to the transition of deformation mechanism from deformation-induced twinning to martensite phase transformation. As a result, excellent balance of strength and ductility was achieved at both temperatures. This finding highlights the importance of SFE for cryogenic alloy design.