The ion irradiation induced crystallization of Ni-Mn-Sn ferromagnetic shape memory alloy (FSMA) thin film is investigated. Thin films of Ni-Mn-Sn FSMA synthesized by DC magnetron sputtering on Si substrate at 200 °C are irradiated by a beam of 120 MeV Ag ions at different fluence varying from 1 x 1012 to 6 x 1012 ions/cm2. X-ray diffraction (XRD) pattern reveals that the pristine film grows in L21 cubic austenite phase with poor crystallinity and crystallinity of the film improves with increasing ion fluence, which is attributed to the strain relaxation by the energy deposited by incoming ions and promotes the grain growth. Grain growth is further confirmed by Atomic force microscopy (AFM). The temperature dependent magnetization measurements show improvement in the magnetic and shape memory properties of the films with increasing fluence, which is ascribed to the ordering of austenite phase. Nanoindentation measurements show that with increasing fluence of 120 MeV Ag ions, films exhibit a greater stiffness and smaller tendency towards plastic deformation.