We studied the spin dependence of conductance and magnetoresistance for clean ferromagnet/graphene/ferromagnet junctions by making use of both simple and realistic band models for ferromagnetic leads. We found that conductance decays monotonically with increasing graphene length for pure graphene, but oscillates for doped graphenes. Magnetoresistance had non-zero values when ferromagnetic leads with a realistic band structure were attached. For doped graphene magnetoresistance also oscillates with graphene length. The oscillation period was determined by the Fermi wave length of the doped graphene.