We describe the synthesis of novel Co9S8-nanowire-filled carbon nanotubes (CNTs) by a simple methodinvolving the pyrolysis of thiophene on cobalt catalyst in a conventional chemical vapor decomposition system.The encapsulated Co9S8 nanowires are single-crystalline, and their lengths are about 10 μm with their [110]direction parallel to the axis of the CNTs. Detailed investigation suggests that the filling of the Co9S8 nanowiresresults from the volume increase induced by a phase transition from cobalt to cobalt sulfide together with thein situ extruding action of CNTs as nanomolds. A new filling mechanism is thus found and proposed.