Usually Ni_clusters are served as catalyst in the formed processo f the single_wall carbon nanotubes by CVD. Because the different surfaces of Ni_clusters have different activity,carbon atoms adsorbed in Ni_clusters (liquid states)will be diffused from low_activity surfaces of Ni_cluster to high_activity ones and be deposited. As a result,the caves of carbon atoms surrounding the Ni_clusters are formed and the single_wall carbon nanotubes are produced. In addition,Ni_atoms served as substitutional atoms always located in the furthest outside of the network of single_wall carbon nanotubes are produced. In addition, Ni_atoms served as substitutional atoms always located in the furthest outside of the network of single_wall carbon nanotubes,and play a role of stabilization in growth of nanotubes,and promot their growth. Therefore,the catalytic mechanism of Ni_clusters as catalyst is introduced,and different morphologies of single_wall carbon nanotubes are also exlained.