The conductances of two typical metallic graphene nanoribbons with one and two defects are studied using the tight binding model with the surface Green＇s function method. The weak scattering impurities, U - 1 eV, induce a dip in the conductance near the Fermi energy for the narrow zigzag graphene nanoribbons. As the impurity scattering strength increases, the conductance behavior at the Fermi energy becomes more complicated and depends on the impurity location, the AA and AB sites. The impurity effect then becomes weak and vanishes with the increase in the width of the zigzag graphene nanoribbons （150 nm）. For the narrow armchair graphene nanoribbons, the conductance at tile Fermi energy is suppressed by the impurities and becomes zero with the increase in impurity scattering strength, U 〉 100 eV, for two impurities at the AA sites, but becomes constant for the two impurities at the AB sites. As the width of the graphene nanoribbons increases, the impurity effect on the conductance at the Fermi energy depends sensitively on the vacancy location at the AA or AB sites.
