A realistic dynamics simulation study is reported for the photoisomerization of trans-azobenzene. The isomerization follows both n pi* (the HOMO -> LUMO) and pi pi* (the HOMO- 1 -> LUMO) excitations. The simulation finds that for the pi pi* excitation, the relaxation of the S(pi pi*) state is immediately followed by double excitation, (pi)(2)(pi*)(2). The decay from the S((pi)(2)(pi*)(2)) state to the S-0 state can occur at partially twisted structure, which favors the formation of the trans isomer. Multiple decay channels are found at about twisted structure for both n pi* and pi,pi* excitations. Decay at about twisted geometry leads to the formation of either cis or trans isomer. Opening of the decay channel at partially twisted structure accounts for the smaller isomerization yield for the pi,pi* excitation.