The dissociation of an As-4 molecule in vacuum was studied. At first, we calculated the energy for the dissociation of one As-4 molecule without the restriction of symmetry and it is found that the energy barrier of the reaction path is smaller than that reported previously. Next. we calculated the dissociation of an As-4 molecule under interaction with another As-4 molecule and found that this energy barrier is smaller than that of the former. This means that at high As-4 pressure, two As-4 molecules react and give As-2 molecules which agrees with the model given by Foxon and Joyce in molecular beam epitaxy (MBE) of GaAs.