  We investigate the electron transport through a graphene p-n junction under a
perpendicular magnetic field. By using Landauar-Buttiker formalism combining
with the non-equilibrium Green function method, the conductance is studied for
the clean and disordered samples. For the clean p-n junction, the conductance
is quite small. In the presence of disorders, it is strongly enhanced and
exhibits plateau structure at suitable range of disorders. Our numerical
results show that the lowest plateau can survive for a very broad range of
disorder strength, but the existence of high plateaus depends on system
parameters and sometimes can not be formed at all. When the disorder is
slightly outside of this disorder range, some conductance plateaus can still
emerge with its value lower than the ideal value. These results are in
excellent agreement with the recent experiment.
