In recent years, increasing interest has been paid to the accurate measurement of the entire tensile stress-strain curve of materials for a wide range of strain rates on the order of 10(-3)similar to10(3) s(-1). Such data are needed to formulate precise constitutive equations for strain rate sensitive materials. The entire tensile stress-strain curves of a cold rolled high strength steel sheet (SPFC440), which due to its improved formability is widely used for automobile structures, and a cold rolled mild steel sheet (SPCE), for a wide strain rate range on the order of 10(-3)similar to10(3) s(-1) were obtained by using the Sensing Block Type High Speed Material Testing System. Based on an investigation of experimental data, strain rate sensitivities of flow stresses of these materials were clarified for the wide strain-rate range. The values of the true fracture strength and the true fracture strain were obtained. It was also found that the value of the fracture strength and the fracture strain are substantially not affected by the testing strain rates.