Crack　propagation　in　sputter-deposited　freestanding　nanocrystalline　tungsten　films　with　a　thickness　of　38　nm　was　studied　using　an　in　situ　transmission　electron　microscopy　(TEM)　tensile　technique.　Distinct　from　coarse-grained　tungsten,　a　totally　intergranular　fracture　mode　and　toughening　mechanisms　were　identified　in　the　nanocrystalline　tungsten.　The　transition　of　the　fracture　mode　indicates　that　the　plasticity　is　improved,　and　the　improvement　may　be　attributed　to　the　effect　of　nano-size　grains　and　the　freestanding　surface,　which　facilitate　a　high　percentage　of　grain　boundaries　(GB)　and　a　weak　binding　force.　A　theoretical　model　of　the　energy　release　rate　and　the　deflection　angle　of　cracks　was　established　to　quantitatively　characterize　the　preferred　deflection　angle.　The　model　predictions　are　in　good　agreement　with　the　experimental　findings.　(C)　2014　Elsevier　B.V.　All　rights　reserved.