Plastic strain distributions in large deformation were obtained at the net section of the central notch of cylindrical specimens with three identical circumferential U-notches spaced longitudinally with an equal spacing. These strain distributions were numerically calculated from the distributions of the hardness number at the net section. Each specimen was loaded up to Pmax (maximum tensile load) and beyond Pmax. In the central notch of the notched specimens subjected to strong effect of notch interference, the axial strain at the center of the net section was greater than that at the notch root and was maximum in the net section. The shape of plastic region in the longitudinal section was an X-shape, which is formed by connecting diagonally two notch roots situated on both sides of the central notch. This shape of plastic deformation remained as the principal plastic deformation up to Pmax, although it became weak with increasing deformation. Beyond Pmax, plastic deformation occurred mainly in the vicinity of the net section of the central notch, causing unstable deformation.