In this paper, crystal damage on TEM sample sidewalls induced by FIB milling during sample preparation was studied. A novel method was invented to prepare the sample, which facilitates the direct observation of amorphous layers on the sidewall. The ion beam acceleration voltage is the dominant factor that affects the damaged layer thickness. The measured amorphous thickness is about 23 nanometers at 30Kv and 10 nanometers at 10Kv. The damage layer thickness is constant with different beam currents over the range from 30pA to 1000pA. Amorphous layer thickness also stays constant with the sample tilt angle.