Thermal decomposition of two kinds of hexanitrohexaazaisowurtzitane（HNIW）particles (230 μm,40 μm) were studied by TG-MS. The isothermal decomposition residues of HNIW with larger particle (230 μm) were researched by FTIR spectra and elemental analysis. The results in the non-isothermal decomposition show that there are two steps for HNIW with larger particle and only one step for HNIW with small particle, the main products are NO,N₂O,CO₂, a small amount of NO₂ and C₂N₂H₂. Isothermal decomposition products of HNIW with large particle at 204 ℃ are same as in the first step of the non-isothermal decomposition. The average molecular formulas of the residues at 204 ℃ for 10 minutes and 60 minutes are C₃H_4.5N_5.5O,C₂H₄N₂O. The total of isothermal decomposition reaction at 204 ℃ is C₆H₆N₁₂O₁₂=6NO+N₂O+2CO₂+2HCN+C₂H₄N₂O.