Double-walled carbon nanotubes (DWCNTs) were synthesized in a packed bed reactor (PBR) and a fluidized bed reactor (FBR) by cracking CH4 oil a Fe/MgO catalyst. It is observed that the dominant carbon product changes drastically from DWCNTs to multi-walled CNTs along the axial direction of PBR. The studies indicated that the high concentration of H-2 from the high conversion of CH4 causes the quick reduction and sintering of the iron catalyst and inhibits the nucleation of DWCNTs. Based on these results, the batch or continuous feeding mode of small amounts of catalyst was adopted in a FBR to maintain a high space velocity of CH4 and to inhibit the negative effect of excess H-2. Finally, a DWCNT product with a specific surface area of 950 m(2)/g and a purity of 98 %, was obtained.