We found that the martensitic transformation in Mn50Ni 33.5Sn8Co8.5 alloy is very sensitive to the surface stress produced during grinding. The grinding-induced martensite in a Mn50Ni33.5Sn8Co8.5 alloy is found to be more thermally stable than the thermally-induced martensite, as indicated by a large increase in the reverse transformation temperature and its latent heat. Annealing the stress-induced martensite was found effective in restoring the austenite structure. The stress-induced martensite was also found to exhibit much enlarged coercive force of 3 kOe, compared to 330 Oe for the thermally-induced undeformed martensite. This was attributed to the pinning of domain walls by internal stress. These observations were noted in contrast to that observed in most NiMn-based ferromagnetic martensitic alloys. © 2013 Elsevier B.V.