The heats of formation (HOFs) for a series of tetrazolo-[1,5-b]-1,2,4,5-tetrazine (TETZ) and 1,2,4-triazolo-[4,3-b]-1,2,4,5-tetrazine (TTZ) derivatives were studied by using density functional theory. The results show that the substitution of the -N(3) or -N(NO(2))(2) group in the TETZ or TTZ ring extremely enhances its HOF values. For monosubstituted case, attachment of a substituent to position 8 in the TETZ or TTZ ring will increase its energy gaps except for the derivatives with the -NO(2) group. It is also found that the energy gap of TTZ can be tuned by incorporating a substituent into different positions in the parent ring. The substitution of the -NH(2) group in the TETZ ring is favorable for enhancing its thermal stability. For the TTZ ring, different substituted positions and number of the substituent might affect its thermal stability. The calculated detonation properties indicate that incorporating the -NO(2), -NF(2), -ONO(2), or -N(NO(2))(2) group into the TETZ or TTZ ring is very helpful for enhancing its detonation performance. Considered the detonation performance and thermal stability, four derivatives may be regarded as the promising candidates of high-energy density materials (HEDMs).