Recent emergence of high entropy alloys (HEAs), which are alloys composed of five or more alloying elements with nearly equimolar concentration, has greatly expanded the possibility of alloy design with outstanding mechanical properties. In addition, medium entropy alloys (MEAs) composed of four or fewer alloying elements with nearly equimolar concentration have been found to exhibit similar or even superior properties compared with HEAs. For example, CoCrNi MEA with FCC single phase showed better strength-ductility balance than that of CoCrFeMnNi HEA due to its higher work-hardening ability, which has been attributed to deformation twinning. In our previous report, we fabricated equiatomic CoCrNi MEA with fully-recrystallized microstructures having a wide range of mean grain sizes through a severe plastic deformation process and subsequent annealing and investigated their mechanical properties. It was found that grain refinement was effective to enhance the yield strength, and, interestingly, the MEA did not lose its high work-hardening ability even after grain refinement down to ultra-fine grain sizes (< 1 um). The reason for this abnormal grain size dependence of the work-hardening ability is still unclear due to the lack of systematic studies on the evolution of deformation microstructures in the MEA. Thus, the purpose of the present work is to reveal the microstructural origin of the high work-hardening ability in ultra-fine-grained (UFG) CoCrNi MEA. Dislocations piling up at grain boundaries were observed immediately after yielding in the MEA with a mean grain size of 16 um, and deformation twinning in multiple twinning systems was observed at a later stage of deformation prior to necking. On the other hand, in the present material with the UFG microstructure, widely-extended stacking faults adjacent to grain boundaries and randomly-tangled dislocations were frequently observed after yielding. After a nominal strain of 3%, deformation twins with a thickness of several nm started forming, in addition to a rapid increase in the density of dislocations and stacking faults. In this presentation, the deformation mechanism which is responsible for the high work-hardening ability in the UFG CoCrNi MEA will be discussed by considering the effect of grain boundaries.