This study investigated the effect of Co substitution for Ni in Mn50Ni42Sn8 alloy with the aim of increasing the magnetic driving force for inducing its martensitic transformation. The martensitic transformation temperatures, enthalpy and entropy changes were found to decrease progressively with increasing Co content, while the transformation hysteresis increased. Co substitution for Ni also significantly increased the magnetization of the austenite, but with negligible effect on that of the martensite. A large magnetization difference 109 emu g(-1) was achieved across the transformation in a Mn50Ni34Sn8Co8 alloy. The large magnetization difference between the two phases provides enhanced thermodynamic driving force for the transformation. Consequently, the martensitic transformation was induced by the application of a magnetic field in Mn50Ni36Sn8Co6 and Mn50Ni34Sn8Co8 alloys. The effect of Co substitution for Ni on the magnetic interaction among the constituents for the austenite and martensite was clarified in this study, which provides a guide for alloy design for magnetoactuation applications.