In order to evaluate 2,4,6-trinitro-1,3.5-triazine (TNTAz), 3,6-dinitro-1,2,4,5-tetrazine (DNTAz), and 2,5,8-trinitro-tri-s-triazine (TNTsTAz). the geometries of these compounds have been fully optimized employing the B3LYP density functional method and the AUG-cc-pVDZ basis set. The accurate gas phase enthalpies of formation have been obtained by using the atomization procedure and designing isodesmic reactions in which the parent rings are not destroyed. Based on B3LYP/AUG-cc-pVDZ calculated geometries and natural charges. the crystal structures have been predicted using the Karfunkel-Gdanitz method. Computed results show that there exists extended conjugation over the parent rings of these compounds. More energy content is reserved in DNTAz than in both TNTAz and TNTsTAz. The title compounds are much more sensitive than 1,3,5-trinitrobenzene. The calculated detonation velocity of DNTAz reaches 9.73-9.88 km s(-1), being larger than those of CL-20 and TNTAz. TNTsTAz has no advantage over the widely used energetic compounds such as RDX and HMX.