Abstract: In situ tensile test of brass foil specimens in TEM shows that many dislocations were emitted from a loaded crack tip and a dislocation free zone (DFZ) was formed under condition of constant displacement. The DFZ is an elastic zone, where the stress may equal to the cohesive strength th when the applied stress is high enough. As a result, nanocracks would initiate discontinuously in the DFZ or, sometimes, at a blunted-crack tip. As soon as the nanocrack nucleated, it quickly blunts into a void, which results in ductile fracture.