We report first-principles calculations of the linear and nonlinear optical susceptibilities of 3-phenylamino4-phenyl-1,2,4-triazole-5-thione crystals. The X-ray diffraction structural data of Wang et al. (Molecules 2009, 14, 608.) was used as the starting point of the computational optimization calculation by minimization of the forces acting on the atoms, and the optimized geometry was used to calculate the linear and nonlinear optical susceptibilities. We have employed the full potential linear augmented plane wave (FPLAPW) method within density functional theory (DFT) along with the Engel-Vosko exchange correlation potential. The full potential calculations show that this crystal possesses an indirect energy gap of 3.1 eV. The compound exhibits some uniaxial dielectric anisotropy resulting in a strong birefringence. The 3-phenylamino-4-phenyl-1,2,4-triazole5-thione crystal possesses high second harmonic generation with several nonzeroth components, but only one component, chi((2))(III)(omega), is dominant and its second-order optical susceptibility of the total real part and the total absolute value at zero frequency is equal to 0.097 x 10(-7) esu.