Electro-polished plate specimens of fully annealed commercial pure iron were tested under completely reversed bending stress at 60Hz, until fine cracks were found on the specimen surface, and the orientation analysis of all the grains, more than 200, in the test area of the specimen surface was made by the etch pit method developed in the authors' previous study. The purpose was to find out the crystallographic and mechanical conditions satisfied at crack nucleation sites. All the cracks were found to nucleate at grain boundaries, and it was concluded that at least eight conditions were satisfied at these sites, with respect to grain size and geometrical configuration of preferential slip systems with grain boundary.
