Study on the thermal instability of N,N'-Dinitrourea (DNU) is very important for its use as an energetic precursor. Thermal decomposition mechanism of DNU is studied by an in situ pyrolytic Fourier transform infrared spectroscopy with the temperature program and density functional theory calculations at B3LYP level. The experimental results have shown that the nitryl group of DNU molecule is initially broken, followed by the rearrangement or reaction simultaneously. All the decomposition reactions are nearly complete prior to 100 degrees C resulting in the formation of N2O, NO2, CON, CONH and NO2NH2. Theoretical calculations confirmed that the syn, anti conformation play a key role in the decomposition process and the N-N bonds were broken firstly with the increase of temperature. The plotted potential energy curve via the corresponding bond length showed that the cleavage of N-N bond in the condensed-phase of DNU was a highly reversible process and a typical radical recombination reaction with zero barriers in the initial step. Finally, the possible decomposition pathways of the N2-N5 bond homolysis and its following reactions were proposed. The final products of N2O, NO2, CONH and NO2NH2, detected in the experiments, were formed by alpha-cleavage or hydrogen atom rearrangement of the radicals that were formed in the initial step. (C) 2011 The Combustion Institute. Published by Elsevier Inc. All rights reserved.