Femtosecond time-resolved transient grating(TG) technique was employed to get insight into the photodissociation mechanism of liquid nitromethane (NM). Broadband white-light continuum was introduced as the probe to observe the evolution of electronic excited states of NM molecules and the formation of photodissociation products simultaneously. The reaction channel of liquid NM under 266 nm excitation was obtained that NM molecules in excited state S-2 relax through two channels: about 73% relax to low lying S-1 state through S-2/S-1 internal conversion with a time constant of 0.24 ps and then go back to the ground state through S-1/S-0 internal conversion; the other 27% will dissociate with a time constant of 2.56 ps. NO2 was found to be one of the products from the experimental TG spectra, which confirmed that C-N bond rupture was the primary dissociation channel of liquid NM. (C) 2016 Elsevier B.V. All rights reserved.