Density functional theory has been used to predict the geometry, electronic structure, harmonic vibrational frequency, explosive property, and azido-tetrazole isomerism of triazidotri-s-triazine at the B3LYP/aug-ccp-VDZ level of theory. Calculation results show that triazidotri-s-triazine molecule keeps a planar structure and there exists considerable conjugation over the molecule, which is advantage to the stability of this compound. Our study shows that triazidotri-s-triazine may be a good explosive. The azido-tetrazole isomerism of triazidotri-s-triazine is investigated in details. The reaction proceeds initially through loss of the linearity of the azido group, approaching the terminal nitrogen N8 atom of the azide group to the nitrogen atom N1 (or N3) of the ring, and this step is then followed by the attack of the lone pair on N1 (or N3) to the azido group, leading to formation of the bond between NI (or N3) and N8. The bending of the N-N-N angle in the azide and the redistribution of electron density associated with these events give rise to a large free energy barrier.