Since their discovery in 1990, the study of sp(2) bonded carbon nanotubes has grown into a field of research in it's own right; however the development of the sp(3) analog, diamond nanowires, has been slow. A number of theoretical models have been proposed to compare the relative stability of diamond and graphite at the nanoscale; and more recently, to compare nanodiamonds and fullerenes. Presented here is a study of the phase stability of nanocarbon in one-dimension. The structural energies of carbon nanotubes and diamond nanowires have been calculated using density functional theory within the generalized gradient approximation, and used to determine the atomic heat of formation as a function of size. (C) 2004 American Institute of Physics.