Electrospun carbon-silicon composite nanofiber is employed as anode material for lithium ion batteries. The morphology of composite nanofiber is optimized on the C/Si ratio to make sure well distribution of silicon particles in carbon matrix. The C/Si (77/23, w/w) nanofiber exhibits large reversible capacity up to 1240 mAh g(-1) and excellent capacity retention. Ex situ scanning electron microscopy is also conducted to study the morphology change during discharge/charge cycle, and the result reveals that fibrous morphology can effectively prevent the electrode from mechanical failure due to the large volume expansion during lithium insertion in silicon. AC impedance spectroscopy reveals the possible reason of unsatisfactory rate capability of the nanofiber. These results indicate that this novel C/Si composite nanofiber may has some limitations on high power lithium ion batteries, but it can be a very attractive potential anode material for high energy-density lithium-ion batteries. (C) 2010 Elsevier B.V. All rights reserved.