Starting with the X-ray diffraction data on the 3-phenylamino-4-phenyl-1,2,4-triazole-5-thione compound obtained by Wang et al. [Molecules, 2009, 14, 608], we have optimised the atomic positions by minimization of the forces acting on the atoms using a full potential linear augmented plane wave method within density-functional theory along with the generalized gradient approximation (GGA) by Perdew, Burke and Ernzerhof (PBE) exchange–correlation potential. In addition, for the electronic band-structure and the calculation of the gap the Engel–Vosko (EV-GGA) scheme has applied. The full potential calculations show that the valence band maximum (VBM) is located at the Γ point and the conduction band minimum (CBM) is located at the Z point of the Brillouin zone resulting in an indirect energy gap of 3.1 eV. The upper VBM is mainly formed by S-p states while the lower CB has mainly C-p states character. Thus the S-p states in the upper valence band and the C-p states in the lower conduction band have a significant effect on the energy band gap. We present an analysis of the partial densities of states which gives useful information on hybridization and the orbital character of the states. We have calculated the bond lengths and bond angles and find better agreement with the experimental data than the density functional theory calculations at B3LYP/6-311G** and PBE1PBE/6-311G** levels of theory by the Berny method within the Gaussian 03 software package. This is attributed to the fact that the values obtained by these methods belong to the isolated molecule in the gas-phase whereas the experimental and the full potential linear augmented plane wave method values are attributed to the molecule in the solid state.