We report the preparation, analysis, and phase transformation behavior of polymorphs of 3-nitro-1,2,4-triazole. The compound crystallizes in two different polymorphic forms, Form I (tetragonal, P41212) and Form II (monoclinic, P21/c). Analysis of the polymorphs has been investigated using microscopy, differential scanning calorimetry, in situ variable-temperature powder X-ray diffraction, and single-crystal X-ray diffraction. On heating, Form II converts into Form I irreversibly, and on further heating, decomposition is observed. In situ powder X-ray diffraction studies revealed that Form II transforms to Form I above 98 °C, indicating that Form I is more stable than Form II at high temperature. Form II of 3-nitro-1,2,4-triazole has good detonation properties (Vdet = 8213 m s-1, PC-J = 27.45 GPa), and low sensitivity (IS > 40 J, FS = 360 N, ESD = 29 J), which make it a competitive candidate for use as a new insensitive explosive.