The influence of pre-deformation on the martensitic transformation behavior and its microstructure has been put forward in a Ni 55 Mn 20 Ga 25 (at.%) ferromagnetic shape memory alloy. It is found that the reverse martensitic transformation temperatures (A s and A f ) increase linearly with increasing amount of deformation, while the martensitic transformation temperatures (M s and M f ) are not changed even when the deformation is up to 6%. However, the effect of the pre-deformation on the reverse martensitic transformation disappears in the following heating process. Transmission electron microscopy shows well-accommodated martensite variants in the undeformed specimen and interfacial boundaries are straight and well-defined. After deformation, the reorientation of martensite variants occurs at the expense of the unfavorable martensite variants, and the interfacial boundary changes from a sharp and straight morphology to a curved and irregular one. Meanwhile, some lattice defects are formed inside the variants even in slightly compressed samples. With increasing deformation, more lattice defects are generated inside the variants as well as in the interfacial boundaries between variants. Based on the analysis of the experimental results, the micromechanism of the effect of pre-deformation on martensitic transformation behavior has been clarified.