Photoisomerization dynamics of s-trans butadiene is investigated by a semiclassical surface hopping trajectory method. The Heisenberg model Hamiltonian is developed to describe two covalent states involved in this process, 1(1)Ag and 2(1)Ag states. This model Hamiltonian well reproduces a global structure of the potential energy surfaces of these states and the nonadiabatic coupling of an ab-initio method. It was found that the nonadiabatic decay from the 2(1)Ag to the 1(1)Ag state takes place at the three partially twisted CC bond conformations. The CCC bending motions largely enhance the nonadiabatic transitions. The internal vibrational relaxation associated with the nonadiabatic transition is also analyzed.