In the previous report, it was investigated to evaluate the thickness effects on brittle fracture toughness of mild steel (SS 41) accompained by large scale plastic deformation ahead of crack tip in the three-point bend tests, which satisfied all aspect of British Standard DD 19. It was showed that the critical COD, Φ_0C value, was dependent on the plate thickness of specimen, and the effects of specimen configuation and ligament length on the Φ_0C value were not yet argued. The object of the investigation is to find the variation of Φ_0C value to the specimen configuration and plate thickness. Two types of specimen, i. e., the three-point bend specimen and the compact tension specimen, of which geometries satisfied BS DD 19 and ASTM E 399 except for the plate thickness, are used in the tests. And the authors present a modification of β-value proposed in the previous report, so that the value may be applied to the evaluation of the effects of plate thickness on the displacement and mouth COD in the specimen. The following conclusions are obtained : (1) The Φ_0C value at which the brittle fracture occurs is dependent on the specimen thickness, and the other dimensions of specimen hardly contribute to Φ_0C value. (2) The thickness effects on brittle fracture toughness should be treated as three dimensional problems, but can be dealt approximately as one of two dimensional problems using the Finite Element Method incorporated the modified β.