We study the work function of low index surface (100) of diamond (face-centered) carbon using pseudopotential plane-wave density functional calculations with the generalized gradient approximation. We consider clean, unrelaxed surfaces in the slab-supercell approximation at 0 K. Particular attention is paid to the convergence of the vacuum in the slab model with respect to slab thickness. The obtained work function, 7.53 eV, shows a slight deviation from the experimental value. Including temperature effect, surface relaxation, and possible surface reconstruction may improve computational results compared to experiments.