Large quantities of high-quality double-walled carbon nanotubes (DWNTs) with very small amounts of amorphous carbon deposits have been produced by catalytic decomposition of n-hexane over Fe-Mo/MgO catalyst. More than 90% of the as-synthesized carbon nanotubes was DWNTs. Raman analysis indicated that the outer tube diameter and inner tube diameter of DWNTs were in the range of 1.44-2.53 nm and 0.70-1.82 nm, respectively, and the interlayer spacing ranged from 0.35 to 0.42 nm. Both Raman analysis and HRTEM observation indicated that the as-synthesized DWNTs were of high quality.