Twin-thickness-controlled　plastic　deformation　mechanisms　are　well　understood　for　submicron-sized　twin-structural　polycrystalline　metals.　However,　for　twin-structural　nanocrystalline　metals　where　both　the　grain　size　and　twin　thickness　reach　the　nanometre　scale,　how　these　metals　accommodate　plastic　deformation　remains　unclear.　Here,　we　report　an　integrated　grain　size　and　twin　thickness　effect　on　the　deformation　mode　of　twin-structural　nanocrystalline　platinum.　Above　a　similar　to　10nm　grain　size,　there　is　a　critical　value　of　twin　thickness　at　which　the　full　dislocation　intersecting　with　the　twin　plane　switches　to　a　deformation　mode　that　results　in　a　partial　dislocation　parallel　to　the　twin　planes.　This　critical　twin　thickness　value　varies　from　similar　to　6　to　10nm　and　is　grain　size-dependent.　For　grain　sizes　between　similar　to　10　to　6nm,　only　partial　dislocation　parallel　to　twin　planes　is　observed.　When　the　grain　size　falls　below　6nm,　the　plasticity　switches　to　grain　boundary-mediated　plasticity,　in　contrast　with　previous　studies,　suggesting　that　the　plasticity　in　twin-structural　nanocrystalline　metals　is　governed　by　partial　dislocation　activities.