Starting from the contact configuration of colliding nuclei, the evolution of the nuclear shape is simulated in two-dimensional deformation space fixing the asymmetry by means of the Langevin equation under the influence of the one body dissipation. It is quantitatively shown that, in heavy mass region, the asymmetry of the entrance channel plays an important role to get an optimum fusion probability in connection with the extra barrier to be overcome. It is necessary to take into account this effect together with the excitation energy at Bass barrier to consider the optimum target-projectile combination for the synthesis of super heavy elements. ©1998 American Institute of Physics.