The changes of microstructures after the exhaustion of dislocation glide ability but before cracking, as well as microcrack initiation by them, were studied in detail by in-situ transmission electron microscopy (TEM) for pure copper, aluminum and iron. Thinning occurred in the thicker regions through dislocation gliding in the three metals. After that, {111} <112> twinning or tensile cracking took place in thinned zones in fcc metals. In the case of twinning, new microcracks were initiated along another {111} plane by the high stress concentration near the growing tip of the twin, while in iron (bcc), many nanocracks were initiated in the thinned areas just ahead of the crack front, resulting in misorientation microstructure from which microcracks or microvoids were developed. The changes of microstructures after the exhaustion of dislocation glide ability but before cracking, as well as microcrack initiation by them, were studied in detail by in-situ transmission electron microscopy (TEM) for pure copper, aluminum and iron. Thinning occurred in the thicker After that, {111} <112> twinning or tensile cracking took place in thinned zones in fcc metals. In the case of twinning, new microcracks were initiated along another {111} plane by the high stress concentration near the growing tip of the twin, while in iron (bcc), many nanocracks were initiated in the thinned areas just ahead of the crack front, resulting in misorientation microstructure from which microcracks or microvoids were developed.