The　fracture　behavior　of　ultrafine　WC-Co　cemented　carbide　was　demonstrated　by　in　situ　tensile　tests　in　a　transmission　electron　microscope.　Using　two　kinds　of　cemented　carbide　samples　with　and　without　previously　introduced　microcracks,　the　process　from　deformation　till　fracture　was　studied.　Particularly,　the　development　and　interaction　of　dislocations　within　Co　and　WC,　as　well　as　the　alternation　of　dislocation　motion　between　the　two　phases,　was　demonstrated,　and　the　features　of　crack　propagation　in　cemented　carbide　were　disclosed.　The　finding　of　the　dislocation　characteristics　and　its　correlation　with　the　fracture　behavior　proposed　the　mechanisms　of　the　ultrahigh　fracture　strength　of　the　cemented　carbide.　[GRAPHICS]　IMPACT　STATEMENT　The　fracture　behavior　of　ultrafine　WC-Co　cemented　carbide　was　for　the　first　time　directly　observed　by　in　situ　TEM　tensile　tests.　The　correlation　of　dislocation　motion　and　fracture　mechanisms　was　proposed.