  The longitudinal resistances of a six-terminal graphene p-n junction under a
perpendicular magnetic field are investigated. Because of the chirality of the
Hall edge states, the longitudinal resistances on top and bottom edges of the
graphene ribbon are not equal. In the presence of suitable disorder, the
top-edge and bottom-edge resistances well show the plateau structures in the
both unipolar and bipolar regimes and the plateau values are determined by the
Landau filling factors only. These plateau structures are in excellent
agreement with the recent experiment. For the unipolar junction, the resistance
plateaus emerge in the absence of impurity and they are destroyed by strong
disorder. But for the bipolar junction, the resistances are very large without
the plateau structures in the clean junction. The disorder can strongly reduce
the resistances and leads the formation of the resistance plateaus, due to the
mixture of the Hall edge states in virtue of the disorder. In addition, the
size effect of the junction on the resistances is studied and some extra
resistance plateaus are found in the long graphene junction case. This is
explained by the fact that only part of the edge states participate in the full
mixing.
