The V-notch Charpy impact test, which has been widely used as a pre-qualification test of material toughness, is also used for estimating "fracture mechanics-based toughness of materials". However, there have not been any theoretical estimation methods to correlate Charpy test results with CTOD fracture toughness, whereas many empirical correlations between these testing results have been suggested. In this research work, we suggest a method for evaluating fracture mechanics based material toughness, which is measured with three-point bend or compact specimens with fatigue pre-cracks, from the Charpy impact test. This method is based on the Weibull stress criterion, in which the Weibull stress at brittle fracture could provide materials constants independent of differences in loading rate and specimen geometry. The applicability of this method is discussed by conducting instrumented Charpy impact tests and CTOD fracture toughness tests in the lower fracture transition-temperature range for two structural steels of the 780-MPa strength class with different work-hardening properties. It is shown that this method makes it possible to transfer the Charpy impact test results to the fracture mechanics-based toughness for two steels with different work-hardening properties.