The microstructure,martensite/austenite(M/A) islands,precipitated phases,high-angle grain boundaries and effective grain size of hot rolled and tempered low carbon bainitic steel AH80 DB were studied by scanning electron microscopy(SEM),transmission electron microscopy(TEM) and electron back-scattered diffraction(EBSD). The reason for the improvement in impact toughness of the low carbon bainitic steel after tempering was discovered. The results show that the microstructures of both the samples mainly consist of lath bainite,granular bainite and acicular ferrite. There is more acicular ferrite in the tempered steel. Coarse M/A islands are found in the hot rolled steel and its distribution is directional,the ratio of large-angle grain boundaries is 17.33%,and the average effective grain size is 3.57 μm. Nevertheless,the size of M/A islands decreases after tempering. Besides,the ratio of largeangle grain boundaries increases by 3.43% and the average effective grain size decreases by 0.56 μm after tempering. The precipitated phase of the hot rolled steel is(Nb,Ti) C with the size of 50-150 nm,while much small and spherical precipitates with the size of about 10 nm appear after tempering.