Double-walled carbon nanotubes (DWNTs) were prepared using floating chemical vapor deposition by adding a small amount of sulfur into the catalyst. The outer diameter of the DWNTs is from 1.5 to 3 nm. Raman study on DWNTs shows that the interlayer distance between the inner and outer tubes varies from 0.33 to 0.41 nm. The as-prepared DWNTs were purified by treatment with H2O2 and HCl to remove the amorphous carbon and catalyst particles from the sample. The carbon content increases from about 30 wt% to above 90 wt% after purification, the great majority of the sample is DWNTs (above 75 wt%).