It is inevitable that high-strength steels of the 50 kg. per sq. mm. class suffer from some embrittlement when they are subjected to bending by the line-heating process. The embrittlement, however, is localized in some limited zone of the plate thickness. Therefore, it is a problem to estimate the notch toughness of the line-heated zone by the Charpy V-notch impact test as generally performed. In this study, an investigation was made to find the behaviour of the full thickness of the plates to the characteristics of brittle fracture initiation (by the Deep Notch test) and crack propagation-arrest (by the Double Tension and ESSO tests) at the line-heated zone, using large-sized test specimens including full thickness plate. The results of the above-mentioned tests are examined about criterions of _v T _rs, ₁₆ T _iC=₁₀, ₁₆ T _{a Kc} =₂₂₀ and ₁₆ T _{a Kc} =₄₃₉. Consequently, having control of cooling condition, the more efficient conditions of line-heating than the actually suggested ones are obtained without the much embrittlement of plate. The line-heating conditions are outlined as follows; 1) In air-cooling after heating, there is no unfavourable effect observed even if the heating temperature is as high as 800 or 900°C. 2) In water-cooling immediately after heating, there is no unfavourable effect observed when the heating temperature is as high as 650°C. 3) In water-cooling during air-cooling after heating, there is comparatively little unfavourable effect observed when the heating, temperature is 800 or 900°C, as long as the temperature at the start of water-cooling is below 500°C.