Carbon nanotubes were obtained by pyrolysis of acetylene or ethylene catalyzed by iron or iron oxide nanoparticles. The morphology, microstructure, and lithium insertion properties of these carbon nanotubes were investigated by transmission electron microscopy, high-resolution transmission electron microscopy, X-ray diffraction, Raman spectroscopy, and electrochemical measurements, respectively. The results showed that the structures of the carbon nanotubes play major roles in both specific capacity and cycle life. Slightly graphitized carbon nanotubes showed a specific capacity of 640 mAh/g during the first charge, whereas well-graphitized carbon nanotubes showed a specific capacity of 282 mAh/g during the first charge. After 20 charge/discharge cycles the charge capacity of the slightly graphitized samples degraded to 65.3% of their original charge capacities, but the well-graphitized samples maintained 91.5% of their original charge capacities. The effects of charge-discharge rates and cycling temperature on lithium insertion properties of carbon nanotubes with different extents of graphitization are discussed.