We carried out first-principle total-energy calculations on carbon nanowires with nanometer-scale diameters up to 4.3 nm consisting of sp(3) carbon atoms. Due to the substantial surface reconstruction, the nanowires are energetically stable although their surfaces are not terminated by any atoms. The calculated total energy of the wire with 4.3 nm diameter is nearly equal to that of the single-walled nanotube with 1 nm diameter. The surface reconstruction results in graphitization of the outermost shell of the wire which is expected to cause the structural transformation from diamond to the multi-shell nanotube or nano-scale graphene ribbons. The electronic structures of the wires are semiconducting with a moderate indirect energy gap. (C) 2009 Elsevier B.V. All rights reserved.