The effect of magnitude and gradient of pre-strain distribution on the notch-brittleness of steel plate for ships was studied. Test pieces were of 450×70×16mm size, centrally notched (Fig. 4), and tested by slow tension. The pre-strains were given by two ways. One is ordinary way (uniform distribution), and its magnitudes were 0% (A-series), 5%(B), 10%(C), 15%(D). Another way was the gradient pre-strain distribution, which are given with a perforated test piece without notch (Fig. 2), and its magnitudes at notch-bottom were 5% (E-series), 10%(F), and 15%(G) The results were obtained as follows: (1) The pre-strain of 5% raises the transition temperature and more pre-strain does not raise the transition temperature (Fig. 8). But the pre-strain lowers the energy level. (2) The gradient of prestrain distribution has little effect on the transition. (Fig. 9) (3) Comparing with the experiment of Prof. Boodberg, the transition curve are sharper, and a Japanese ship plate (rimmed) showed the transition temperature of the same order with Prof. Boodberg's “ C ”-steel (semi-killed) after correction of r/t (Fig. 10), where r, t are the radius and thickness of test piece.