The chemistry and physics of the formation and evolution of condensed carbon phases are very important aspects of nitramine detonation. The observation of the synthesis of ultrafine diamond in detonation soot has initiated numerous studies to understand its kinetics and influence on various detonation phenomena such as the detonation build-up of carbon-rich explosives. However, at present it is still very difficult to directly measure the compositions and phase evolution of reaction products during detonation because of the extremely high temperature, pressure and short duration of the process. In order to determine the formation of condensed carbon phases in energetic materials, we focus on study of the initiation mechanisms of their decomposition and formation of new molecules under various thermal conditions. In this work, we report studies of the thermal decomposition of energetic materials TATB and HMX under different thermal conditions from molecular dynamics (MD) simulations with the first-principles based ReaxFF reactive force field. Simulation results show that no significant reactions or related cluster formation is observed in either HMX or TATB at lower temperatures below 1200K, while at higher temperatures reactions are initiated in both materials, but the decomposition rate for TATB is significantly lower than for HMX. We also found that TATB decomposition leads to formation of much larger carbon clusters than does HMX and that the size of the largest cluster begins earlier for higher temperatures. The structure of the carbon rich phase formed from TATB shows that it contains mainly graphitic sheets. This is consistent with the theoretical carbon phase diagram for small system sizes. Thus, the ReaxFF-MD simulations provide atomistic insight into the kinetics of carbon cluster formation and initial chemical events during thermal-induced decomposition of the energetic materials.