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Thermomechanical process and tempering heat treatment were employed to produce the experimental steel plates. The effect of boron addition on the microstructure and mechanical properties of low carbon bainitic steels was studied in this paper. Microstructure observation and crystallographic features were conducted by using optical microscopy, SEM, TEM and electron back scattering diffraction (EBSD) analysis. The results showed that under the same rolling processes and heat treatment conditions, a substantial increase in strength is obtained by addition of boron into steel, but accompanied by an obvious drop in toughness. New martensite phase forms along the grain boundaries on tempering at 650 ℃ mainly due to boron segregation, which can further deteriorate impact toughness of the boron bearing steel. The EBSD analysis showed that high angle grain boundary is not responsible for the deteriorated toughness of the boron bearing steel because it has relatively higher percentage of high angle grain boundary than the boron free steel. The low toughness of the boron bearing steel is mainly attributed to the coarse boride precipitated particles according to the results of fractograph observation.