A critical experiment is presented for the mechanisms of initiation and propagation of cleavage crack in iron. At first, precise fractographic observations are made on the type of the origin of crack observed on single crystals of iron with notch. A number of cleavage facets are started from the boundary of a twin, and a straight groove runs through close to the center of the twin. Striations cross the twin with the appearance of shear fracture inside the twin. A possible interpretation for the process of fracture is presented. Secondary, the arresting phenomenon of the fast propagating cleavage crack, together with the nature of plastic deformations produced near the surface of the crack, is studied on the plate of single crystals of iron with large temperature gradients. The surface of cleavage is characterized by the high density of fine twins in the region of low temperatures −196 to 0 C and the apparent smooth surface lacking twins in the regions of high temperatures 100 to 300 C. Etch pit studies, however, revealed the high density of dislocations (109 to 1010 /cm2) confined within a layer of about 3 μm in the later region. From these evidences it is concluded that the nucleation of dislocation loops is a more dominant cause in the plastic deformations than the multiplication of dislocations from the pre-existing dislocations near the crack tip.