We investigated the grain size effect on martensitic transformation behavior in Fe-30at.%Ni powder and ribbon specimens. The powder specimen with a particle size of 5 um does not show an athermal martensitic transformation but does show an isothermal martensitic transformation after an incubation time of about 104 s at 205 K. On the other hand, the powder specimen with a particle size of 20 um shows an athermal martensitic transformation at 150 K. The value of Ms is much lower than that of the single crystal and of bulk specimens. However, the Ms temperature of a ribbon specimen with an average grain size of 15 um is found to be almost identical to that of the single crystal and of bulk specimens. Considering these results, the athermal martensitic transformation is suppressed by the decrease in particle size if grains do not have grain boundaries. © 2009 IOP Publishing Ltd.