High-purity carbon nanotubes(CNTs)were synthesized in hydrogen ambience by CVD method by (using) rare earth alloy MlM_(5-1.35)(CoAlMn)_(1.35) as catalyst, acetylene as carbon source. The influence of hydrogen flow rate on the diameter, shape, purity, graphitization degree and output of carbon nanotubes was systematically studied by scanning selectronic microscopy(SEM), transmission electron microscopy(TEM), X-ray diffraction(XRD)and Raman technique. The results indicate that the size of MlM alloy particles changed from μm order into nm order during the process of hydrogen reduction. While no introducing hydrogen in the reaction of acetylene and catalyst, CNTs can also be synthesized on the catalyst. The CNTs are short and thick with average diameter of about 97.8 nm, and the wall of CNTs is also thick and rough. With increasing of hydrogen flow rate, the diameter of CNTs first decrease and then increase, while the output, purity, and graphitization degree first increase and then decrease. The optimized conditions are: CNTs reach its narrowest size with diameter of 49.1 nm when hydrogen flow rate is 50 ml·min~(-1). The purity, graphitization degree, and output of CNTs reach highest and largest when hydrogren flow rate is 75 ml·min~(-1). It has the most uniform tube size of 97.38 nm when hydrogen flow rate is 100 (ml·min~(-1).) High-purity carbon nanotubes (CNTs) were synthesized in hydrogen ambience by CVD method by (using) rare earth alloy MlM_ (5-1.35) (CoAlMn) _ (1.35) as catalyst, acetylene as carbon source. on the diameter, shape, purity, graphitization degree and output of carbon nanotubes was systematically studied by scanning selectronic microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and Raman technique. The results indicate that the size of MlM alloy particles changed from μm order into nm order during the process of hydrogen reduction. While CNTs are short and thick with average diameter of about 97.8 nm, and the wall of CNTs is also thick and rough. With increasing of hydrogen flow rate, the diameter of CNTs first decrease and then increase, while the output, purity, and graphitization degree first increase and then decre decrement a. The optimized conditions are: CNTs reach its narrowest size with a diameter of 49.1 nm when the hydrogen flow rate is 50 ml · min -1. The purity, graphitization degree, and output of CNTs reach highest and largest when hydrogren flow rate is 75 ml · min ~ (-1). It has the most uniform tube size of 97.38 nm when hydrogen flow rate is 100 (ml · min -1).