Anhydrous 5-aminotetrazole (5-amino-1H-tetrazole, 5-ATZ) is an energetic material that produces a large amount of nitrogen gas by thermal decomposition and is often used as a gas generator agent. Although it is used widely as an air bag inflator, its crystal structure has not been determined yet. Thus, we performed a powder X-ray diffraction experiment, a Rietveld analysis, and the density functional theory calculation to investigate its structure and determined it to be an orthorhombic P2₁2₁2₁ with a 1H-form molecule. We also investigated the high-pressure behavior and found that the orthorhombic phase is stable up to at least 11.6 GPa. Furthermore, the quantum molecular dynamics calculations at high-temperature and high-pressure of 5-ATZ were carried out and predicted the phase change including molecular decomposition.