Nanosized Fe2O3 particles (nano-Fe2O3) with two shapes (tetrakaidecahedral and grainy) were synthesized by hydrothermal methods. The morphologies and structures were characterized using a combination of experimental techniques including X-ray diffraction (XRD) and scanning electron microscopy (SEM). Two composites containing CL-20 (hexanitrohexaazaisowurtzitane, HNIW) and tetrakaidecahedral nano-Fe2O3 [nmT-Fe2O3/CL-20] or grainy nano-Fe2O3/CL-20 (nmG-Fe2O3/CL-20) were prepared. The thermal behaviors of the two composites and pure CL-20 were investigated using differential scanning calorimetry (DSC). Non-isothermal decomposition kinetic parameters and the thermal decomposition mechanism of the two composites and CL-20 were obtained. The apparent activation energy (E-a) of the main thermal decomposition reaction of CL-20, nmT-Fe2O3/CL-20 and nmG-Fe2O3/CL-20 are 181.94, 179.17, and 176.18kJmol(-1), respectively. The thermal decomposition mechanism of CL-20 as well as nmT-Fe2O3/CL-20 was controlled by the Avrami-Erofeev equation (n=2/5) assumed as random nucleation and subsequent growth, while, the reaction mechanism of the composite nmG-Fe2O3/CL-20 was controlled by the Mample Power law (n=1/2). The reason for this difference may be due to the different morphology and particle size of the two nano-Fe2O3 particles.