The effect of irradiation on the martensitic transformation of TiNi shape memory alloys was studied by transmission electron microscopy (TEM) with in situ observation and differential scanning calorimeter (DSC). The results of TEM and DSC measurements show that the microstructure of samples is R phase at room temperature. Electron irradiations were carried out using several different TEM with accelerating voltages of 200, 300, 400 and 1000kV. Also the accelerating voltage in the same TEM was changed to investigate the critical voltage for the effect of irradiation on phase transformation. It was found that a phase transformation occurred under electron irradiation above 350kV, but never appeared at 340kV or lower accelerating voltage. Such phase transformation took place in a few seconds of irradiation and was independent of atom diffusion. The mechanism of electron-irradiation-induced the martensitic transformation is due to displacements of atoms from their lattice sites produced by the acceleratedelectrons. The critical displacement energy of TiNi SMAs is B18 eV from the results.