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.m^1/2/s. The K_Jc values converted from J_c 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 K_Jc 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 K_Jc 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 K_Jc 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 K_Jc values. The K_Jc values at Δa₀ = 0, X = 0, and Δa₀ + X = 0 are proposed as the lower-bound fracture toughness of a steel, which are labeled K_Jci. The shape of K_Jci versus temperature curve is controlled by the critical stretch zone width, SZWc, which was confirmed by the fact that the K_Jc values are proportional to the amount of SZWc. The Weibull slope m of fracture toughness K_Jc 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 K_Jc values with 3% fracture probability coincide with the K_Jci value in the lower temperature region even in higher strain rate tests.