Well-dispersed nanosized hard carbon spherules (nano-HCS) with different hydrogen contents were prepared by a modified hydrothermal method. Their electrochemical behaviors were investigated. The results from Raman spectra and the HRTEM images at different lithiation states show that the microstructure of the nano-HCS with high hydrogen content becomes slightly ordered after several discharging/charging cycles. This "disorder-to-order" transition phenomenon was not observed in the nano-HCS with low hydrogen content. A mechanism is proposed to interpret this phenomenon. Furthermore, the formation of a thick solid electrolyte interphase (SEI) film on the surface of the nano-HCS is demonstrated and it could be decomposed after charging. (c) 2007 Published by Elsevier B.V.