A　tensile　technique　was　developed　and　coupled　with　in-situ　transmission　electron　microscopy　observations　to　directly　characterize　the　crack　propagation　mechanism　in　sputter-deposited,　ultra-thin,　free-standing　nanocrystalline　Ag　thin　films　with　a　thickness　of　60　nm.　The　developed　technique　directly　revealed　the　fracture　mechanism;　the　thin　film　with　nanoscale　grains　exhibits　ductile　fracture　behavior,　and　the　crack　propagates　through　void　nucleation,　growth,　and　coalescence　ahead　of　the　crack　tip.　A　model　for　the　energy　release　rate　during　the　propagation　of　nanovoids　was　established　to　quantitatively　characterize　the　equilibrium　length　of　the　voids.　Based　on　experimental　measurements　and　theoretical　calculations,　the　effects　of　stress　distribution　and　energy　transformation　on　the　nucleation　position,　equilibrium　length,　and　growth　rate　of　the　nanovoids　are　discussed.　(C)　2014　Elsevier　B.V.　All　rights　reserved.