Defects　in　graphene　nanoplatelets　(GNPs)　and　their　interface　behavior　to　reinforce　magnesium　alloys　was　studied　by　using　two　types　of　GNPs,　one　with　high　integrity　and　the　other　with　multiple　defects,　as　reinforcements　in　ZK60　alloy.　Herein,　GNPs　prepared　by　chemical　exfoliation　exhibited　multiple　defects,　such　as　carbon　atom　vacancies,　dangling　carbon　bonds,　oxygen-containing　functional　groups　and　wrinkles.　During　the　composite　fabrication,　Mg　atoms　restored　the　sp(2)　structure　of　graphene　and　reacted　with　the　oxygen-containing　groups　on　graphene,　leading　to　a　strong　interfacial　bonding.　Meanwhile,　graphene　defects　scattering　in　nanoscale　triggered　nucleation　of　Mg　nanograins,　which　connect　the　＂hard＂　GNPs　and　＂soft＂　Mg　matrix　for　effective　transfer　of　stress　and　strain.　The　defective　GNPs　(0.05　wt%)　reinforced　ZK60　alloy　with　large　tensile　yield　strength　enhancement　(65%)　and　especially　remarkable　elongation　improvement　(44%).　This　work　highlights　the　structural　feature　of　GNPs　to　reach　their　full　strengthening　potential　as　reinforcements　in　metal　matrix　composites.