The thermal isomerization reaction of cis-azobenzene to its trans isomer has been studied by direct molecular dynamics. The semiempirical PM3 method was used to calculate the force field, which was verified by density functional (PW91) calculations. The transition state for the reaction was searched by both methods. The trajectories were started from the transition state dressed with zero-point vibration, and the transient vibrational spectrum was collected. Two types of the vibrational modes were found to be excited at 1 ps, both of which correspond to in-plane skeletal vibration of the benzene ring (nu(8) and nu(19)). The lifetimes of those modes are estimated to be 1.2 and 1.8 ps, respectively. The results indicate that nu(8) and nu(19) modes of the benzene ring play an important role in the thermal isomerization.