Based on first-principles electronic structure and transport calculations, we have studied electronic structure and transport properties of graphene nanoribbons with single vacancy defects. It is shown that introduction of the single vacancy defects leads to a flat band belt at the Fermi energy level for graphene nanoribbons and the semiconductor-metal transition in zigzag semiconducting graphene nanoribbons, which is useful in the energy-band engineering. Armchair graphene nanoribbons with odd width are metallic with good electric conduction while armchair graphene nanoribbons with even width have metallic band structures with character of the group IV semiconductor. Single vacancy defects weakens the conduction of armchair graphene nanoribbons with odd width while obviously strengthens the conduction of armchair graphene nanoribbons with even width.