To improve the understanding of the unimolecular decomposition mechanism of nitroglycerin (NG) in the gas phase, density functional theory calculations were performed to determine various decomposition channels at the B3LYP/6-311G** level. For the unimolecular decomposition mechanism of NG, we find two main mechanisms: (I) homolytic cleavage of O-NO2 to form •NO2 and CH2ONO2CHONO2CH2O•, which subsequently decomposes to form •CHO, •NO2, and 2CH2O; (II) successive HONO eliminations to form HONO and CHO-CO-CHO, which subsequently decomposes to form CH2O + 2CO2 and •CHO + CO. We also find that the former channel has slightly smaller activation energy than the latter one. In addition, the rate constants of the initial process of the two decomposition channels were calculated. The results show that the O-NO2 cleavage pathway occurs more easily than the HONO elimination.