Strength of pulp canal reamers are important problems for cutting treatments of the root of tooth at the dental regions. Reamers have been used as cutting tools by the twisting movements, and then, fracture of reamers are especially caused by the shearing stress at the bite tips. So, this paper treated the shear strength of dental canal reamers, which are torsional strength and twisting rigidity of these cutting instruments. And we made the torsional apparatus for cutting and twisting tests, and tested the torsional strength of raw materials and pulp canal reamers. Type of reamer's configuration are treated by Japanese industrial standard (JIS) and these properties are readjusted with dimensions of diameter of reamers. Shear strength of reamers are 120 approximately 140 kg/mm2 and strength of straight bar with taper are about two times of that of reamers which are about 40 kg/mm2 (except reamer's number 1, 2). Scattering of value of strength and torsional angle of these reamers are examined and showed the range of scattering of these properties. Effectiveness of torsional angle on the fracture properties are considered on the clinical treatments and treated the safety of fracture of these reamers.