We made the tension tests about the specimens of annealed mild steel with various circumferential notches, and made σ-q diagrams about each specimen, where σ indicates the tensile stress and q indicates the area contraction corresponding to every stage of loads during the tension tests. And so we required the relation between the sharpness of the notch and tensile strength σB, breaking stress on final area σT or area contraction ψ.Summary of the results is as follows:(1) Tensile strength σB increases when the notch becomes sharper, and approaches to cirtain maximum value.(2) Area contraction ψ decreases when the notch becomes sharper, and approaches to cirtain minimum value.(3) The ture breaking stress on final area σT increases at first but thereafter it decreases when the notch becomes sharper. For this reason we explained using the stress distribution in the section of the necked portion when the specimen breaks.