Double-walled carbon nanotubes (DWNTs) are irradiated by ultraviolet laser. Scanning electronic microscope, transmission electronic microscope and Raman spectroscopy are used to detect the morphology change in the DWNTs. It was found that carbon onions and Fe particles formed after laser irradiation. The morphology and component of these particles are dependent on the laser energy input. As energy input increased from 100～107 mW·cm-2/mm, carbon onions keep growing, while size of Fe particles firstly grows and then reduces with the turning point at 103 mW·cm-2/mm.