The effect of magnetic fields on the athermal martensitic transformation in an Fe-31.4Ni-0.5Mn alloy (mass%), whose M_s temperature is about 195 K, and on the isothermal one in an Fe-24.9Ni-3.9Mn alloy (mass%), whose nose temperature is about 153 K, have been examined in order to clarify the difference between the athermal and isothermal kinetics of martensitic transformations by measuring magnetic field susceptibility and magnetization, and by observing optical microstructure, applying pulsed ultra high magnetic fields up to 31 MA/m. As a result, the following characteristics were found: The austenitic state in the former alloy was ferromagnetic, whereas that in the latter alloy was micromagnetic and/or spin glass. Even in the latter alloy, martensitic transformation was induced instantaneously under pulsed magnetic fields higher than a critical one over a wide temperature range, as in the former alloy. This result suggests that the originally isothermal kinetics of martensitic transformation is changed to the athermal one under high magnetic fields. Optical microscopy showed that respective isothermal martensite plates of the Fe–24.9Ni–3.9Mn alloy under no magnetic field grew gradually during isothermal holding. Moreover, morphology of the magnetic field-induced martensites of both the alloys was almost the same as that of thermally-induced ones, irrespective of the formation temperature. A thermodynamic calculation for the critical magnetic field vs. temperature relation was done for both the alloys by using an equation previously proposed, and the calculated relations were all in good agreement with the experimental ones in the wide temperature range examined.