In premixed flames of most hydrocarbon fuels, there is only one stage heat release. However, two-stage heat release occurs in premixed nitromethane/air flames under certain conditions. In this study, numerical simulations were conducted for one-dimensional planar and spherical nitromethanelair flames at different initial temperatures (423 similar to 800 K), pressures (0.5 similar to 10 atm) and equivalence ratios (0.5 similar to 4.3). Using the planar flame, we investigated the characteristics of the two-stage heat release and identified elementary reactions involved in these two stages. It was found that the occurrence of two-stage heat release strongly depends on the equivalence ratio and that single-stage heat release occurs for very fuel-lean mixture. To demonstrate the key reactions involved in the second stage heat release, we modified the original chemical mechanism and compared the results predicted by different mechanisms. The second stage heat release was found to be mainly caused by the reaction NO+H -> N+OH. Using the propagating spherical flame, we assessed the impact of two-stage heat release on the determination of laminar flame speed. The positive burned gas speed induced by the second stage heat release was shown to affect the accuracy of laminar flame speed determined by traditional method neglecting burned gas speed and using the density ratio at equilibrium condition. Alternative methods were proposed and used to correct the experimental data reported in the literature. (C) 2017 The Combustion Institute. Published by Elsevier Inc. All rights reserved.