In-situ transmission electron microscopy was applied for clarifying radiation-induced amorphization, the behavior of grain boundaries under ion irradiation. The effect of mass and energy of several ions on preferential amorphization was discussed. The critical fluence for amorphization strongly depended on the temperature, where it increased with increasing temperature. Further, with increasing ion mass and decreasing energy, the critical fluence was reduced. The onset temperature for preferential amorphization increased in the case of heavier mass and lower energy ion irradiation. All of the results imply the importance of the balance between damage production and recovery.