In investigating the brittle fracture initiation characteristics of structural steels, the problem has to be solved whether the laboratory data obtained by experiments using specimens of small size can directly be applied toward predicting the brittle fracture initiation in actual steel structures. A good deal of studies have been made in investigating the properties of low stress level brittle fracture which accompanies very little plastic deformation, which is comparatively easier problem to deal. However from practical view point, it is of more importance to clarify so-called a high stress level brittle fracture which involves a larger amount of plastic deformation. It has been found that the use of COD concept is effective in approaching problems involving large plastic deformation. Authors performed brittle fracture initiation tests using deep notch specimens of different size and discussed the validity of the application of laboratory test data using small test pieces toward larger structures and justified that the critical COD value predicts the initiation of brittle fracture upto around the general yielding stress level, irrespective of the specimen size.