Carbon-nanotube-encapsulated carbon nanofibers (CNFs@CNTs) with a novel one-dimensional structure are synthesized via the selective assembly of CNFs inside the channels of CNTs (see figure). The resulting novel carbon hybrid material serves as stable anode in lithium batteries during 120 charge/discharge cycles. This work provides a simple and efficient way of converting conventional CNTs into functional carbon materials with an outstandingly high volumetric storage density.