Detwinning　is　an　important　plastic　deformation　mechanism　that　can　significantly　affect　the　mechanical　properties　of　twin-structured　metals.　Although　many　detwinning　mechanisms　have　been　proposed　for　pure　metals,　it　is　unclear　whether　such　a　deformation　model　is　valid　for　nanocrystalline　alloys　because　of　the　lack　of　direct　evidence.　Here,　the　atomic-scale　detwinning　deformation　process　of　a　nanocrystalline　AuAg　alloy　with　an　average　grain　size　of　similar　to　15　nm　was　investigated　in　situ.　The　results　show　that　there　are　three　types　of　detwinning　mechanisms　in　nanocrystalline　AuAg　alloys.　The　first　type　of　detwinning　results　from　grain　boundary　migration.　The　second　type　of　detwinning　occurs　through　combined　layer-by-layer　thinning　and　incoherent　twin　boundary　migration.　The　last　one　occurs　through　incoherent　twin　boundary　migration,　which　results　from　the　collective　motion　of　partial　dislocations　in　an　array.