Line-heating is a thermoplastic working technique widely applied to bending and straightening of hull plates in shipbuilding. In this study, the thermoplastic deformation and the local brittleness due to line-heating for high-strength steel of the 50 kg. per sq. mm class were investigated by the Charpy V-notch impact test, hardness test and observation of microstructure. The results obtained are as follows; 1) The relation between the line-heating condition and the thermoplastic deformation was clarified. 2) In the heating at the temperature exceeding the transformation temperature A_c1, the air-cooling causes only little embrittlement. 3) In the water-cooling after heating, the embrittlement is comparatively slight if the starting temperature of water-cooling is kept below the transformation temperature A_r1.