Coupon tension tests for steel plates of SS 41 and SM 50 A are carried out in order to obtain the stress-strain diagrams up to the maximum load. The specimens consist of 36 coupons supplied from 6 mill-makers with blast furnaces in Japan. The thickness of the plates are ranging from 6 mm to 50 mm. Testing methods are compared between JIS (Z 2241) and RILEM (TC83-CUS Testing Recommendation), where the strain rate according to the former is higher than the latter, especially in plastic range. The values of the representative points on the stress-strain curves, for exsample, upper yield strength, lower yield strength, strain-hardening point, tensile strength, rupture load, reduction of area, Young's modulus, Poisson's ratio, some points on the strain-hardening region, etc., are measured. The results are discussed by comparing with the difference of testing methods. The values of the mechanical properties from the inspection certificates (socalled Millsheet values) are also ref. erred. Moreover, mathematical expressions for strain-hardening region of the stress-strain diagrams are tried by applying a modified Menegotto-Pinto model which is proposed in RILEM recommendation. The two material constants including in this model are determined for SS 41 and SM 50 A, respectively. Finally, a standard stess-strain diagram expessesed by the model is given and the scatter or deviation is evaluated.