We investigated the heat of formation (ΔfH) of polynitrocubanes using density functional theory B3LYP and HF methods with 6-31G*, 6-311+G**, and cc-pVDZ basis sets. The results indicate that ΔfH firstly decreases (nitro number m=0–2) and then increases (m=4–8) with each additional nitro group being introduced to the cubane skeleton. ΔfH of octanitrocubane is predicted to be 808.08 kJ mol−1 at the B3LYP/6-311+G** level. The Gibbs free energy of formation (ΔfG) increases by about 40–60 kJ mol−1 with each nitro group being added to the cubane when the substituent number is fewer than 4, then ΔfG increases by about 100–110 kJ mol−1 with each additional group being attached to the cubic skeleton. Both the detonation velocity and the pressure for polynitrocubanes increase as the number of substituents increases. Detonation velocity and pressure of octanitrocubane are substantially larger than the famous widely used explosive cyclotetramethylenetetranitramine (HMX).