Ignition delay times of the nitromethane (NM)/O-2/Ar mixtures were measured behind reflected shock waves in the temperature range of 1200-2000 K, pressures of 2.0 and 10.0 atm, equivalence ratios of 0.5, 1.0, and 2.0, and fuel concentrations of 0.5%, 1.0%, and 2.0%. OH emission intensity and pressure evolution were used for the ignition delay time determination. Two-stage ignition behavior was observed in the experiments. The ignition delay time decreases sharply with pressure increasing and equivalence ratio decreasing. In addition, a sub-model including 24 updated reactions for nitromethane oxidation was proposed based on ab initio calculation with CCSD(T)/CBS//B3LYP/6-311 + G(3DF, 3P) level. The present model and the Mathieu's model (Mathieu et al. Fuel 182(2016): 597) were then validated against present measured ignition delay times, as well as some laminar flame speed data and ignition data in the literature. The present model shows better or comparable predictions with the experimental data. The main species concentration evolution and reaction analysis were presented, based on which the experimentally observed two-stage ignition behavior was interpreted.