Carbon nanotubes capsules (CNCs) with compact, stout walls and tunable sizes were fabricated by using self-assembly of acid modified carbon nanotubes in a water-in-oil emulsion system. The effect of ultrasonic power on the formation and size of CNCs were investigated. On the basis of fabrication of CNCs, CNCs encapsulating SnO2 nanoparticles were prepared as anode material for lithium ion batteries. The morphologies, structural characteristics and electrochemical performances of CNCs and CNCs encapsulating SnO2 nanoparticles were systemically investigated by FE-SEM, TEM. XRD and a series of electrochemical testing techniques. The results showed that the encapsulation amount of SnO2 in CNCs had a great influence on the reversible capacity and cycle performance of the composites. The composite with appropriate amount of SnO2 exhibited a high reversible capacity of 383 mAh g(-1) and an excellent cyclability with only 0.4% capacity loss/cycle in that CNCs; not only could provide high electric conductivity for composites but also effectively accommodate the volume change of SnO2 during the cycling processes. (C) 2009 Elsevier Ltd. All rights reserved.