The mechanisms of the nitrolysis of hexamethylenetetramine (hexamine or HA) with nitric acid, and with the mixture of nitric acid and ammonium nitrate to form 1,3,5-trinitro-1,3,5-triazacyclohexane (hexogen or RDX) were studied by NMR tracing. It was found that HA immediately disappeared, while RDX was formed gradually, which indicates the formation of some intermediates in the HA nitrolysis to give RDX. The chemical shifts of the peaks in the H-1- and C-13-NMR tracing spectra disagreed with the possible cyclic-structure intermediates proposed by Wright and co-workers. Comparison of the C-13-NMR spectra showed that the nitrolysis fragments in K process can be used to form RDX. Based on the results, structures of intermediates were proposed. All the evidences mentioned above suggested a novel mechanism of the formation of RDX from HA i.e., the nitrolysis of HA first gives some open-chain methylenenitramine intermediates, and the condensation of the latter gives RDX under certain conditions.