Static and dynamic fracture toughness tests were performed in the transition region for two ASTM A508 Class 3 steels using 25.4-mm (1-in.) thick compact tension (1T-CT) specimens and precracked instrumented Charpy specimens. The strain rate in terms of K ranged from 1.6 to 6.5 × 105 MPa.m1/2/s. The KJc values converted from Jc of small specimens indicated a wide scatter. When the strain rate increases, the dynamic fracture toughness transition curves shift to higher temperature region, and they give the lower-bound fracture toughness of the steels. Increasing the strain rate reduces the scatter of KJc values dramatically, especially in the higher temperature region and decreases the lower-bound fracture toughness. Fractographic examination of the fractured specimen surfaces indicated that the KJc versus stable crack growth. Δa0, distance from stable crack front to trigger point, X, and distance from fatigue crack front to trigger point, Δa0 + X, relationships construct each single curve even in the case of higher strain rate. The scatter of KJc values is caused by the variance in the amount of Δa0, X, and Δa0 + X. With increasing strain rate, the amount of Δa0, X, and Δa0 + X decreases significantly, which leads to the small scatter of KJc values. The KJc values at Δa0 = 0, X = 0, and Δa0 + X = 0 are proposed as the lower-bound fracture toughness of a steel, which are labeled KJci. The shape of KJci versus temperature curve is controlled by the critical stretch zone width, SZWc, which was confirmed by the fact that the KJc values are proportional to the amount of SZWc. The Weibull slope m of fracture toughness KJc values becomes larger with increasing strain rate and decreasing temperature. Higher toughness data with larger stable crack extension than 100 μm violates the linearity of Weibull plots and makes a bilinear relationship. In the statistical approach to determine the lower-bound fracture toughness in the transition region, much more analytical development is needed. The average of KJc values with 3% fracture probability coincide with the KJci value in the lower temperature region even in higher strain rate tests.