Properties of the circular bar with a rectangular notch are researched, with special reference to tension, intending to clarify the influence produced by the depth and breadth of the notch upon the breaking stress of the several test pieces. In the experiments, steel bar test pieces of 1.5cm dia., having the following 36 types of rectangular notch are stretched out by Olsen-type universal testing machine. Breadth of notch=0.1, 0.2 and 0.3cm. Depth of notch=0.1, 0.2, 0.3, 0.4, 0.5 and 0.6cm. Consequently, the relations between the ultimate strength and the sectional area of notch are obtained, and true breaking stresses are the same with all the test pieces except for circular bar with no notch or with a deep notch.