Density functional method was applied to the study of the isomers of the primary explosive of tetrazene. The geometrical structures of the three isomers of tetrazene molecule which had been proposed formerly were fully optimized at the B3LYP/6311+G^** level of theory. The results show that the total energy of (Ⅲ) is less than that of (Ⅰ) and (Ⅱ),indicating that (Ⅲ) is the most stable structure,which is in agreement with the fact that tetrazene molecules in its crystal adopt structures similar to (Ⅲ). The computational results of the IR vibration of the three isomers show that there is no imaginary frequency,which indicates that all of them are possibly existent structures. The NBO charges of the atoms of the three isomers show that the arrangement of atoms of (Ⅲ) facilitates the charge distribution better than (Ⅰ) and (Ⅱ). It can be concluded from the frontier molecular orbital analysis that N(8) is the most active atom of tetrazene,and bond N(8)—N(12) will break first when it is activated.