Abstract: Disclosed is a lanthanum doping catalyst for preparing carbon nanotubes having uniform diameter and producing method thereof. The catalyst comprises magnesium oxide as a support, two or three metal oxides selected from the group consisting of ferric oxide, cobalt oxide, and nickel oxide as a complex metal oxide composite, lanthanum oxide as a lanthanum doping composite, and molybdenum oxide as an enhanced catalytic composite. The catalyst is produced by dissolving magnesium salt into distilled water, and into the solution is added metal salt for forming complex metal oxide composite, lanthanum salt for forming lanthanum doping composite and salt for forming enhanced catalytic composite in a molar ratio of 0.5-3.0:0.1-1.0:0.01-1.0:0.5-3.0. The solution is dissolved completely and dried at 120-200° C. for 3-5 hours. Then, the product is calcinated at 550-850° C. for 10-30 minutes and grind to be a fine powder.