The purpose of this study was to describe the roles of microstructure types and grain boundary characteristics in fatigue crack propagation behavior in ferrite-pearlite steel and ferrite-bainite steel.The ferrite-bainite dual-phase steel was obtained by intermediate heat treatment conducted on ferrite-pearlite low carbon steel.This paper presents the results from investigation using constant stress-controlled fatigue tests with in-situ scanning electron microscopy(SEM),electron backscattering diffraction(EBSD) and fatigue fractography analysis.Microscopic images arrested by in-situ SEM showed that the second hard bainite phase distributed in the soft ferrite matrix had a significant effect on preventing the cracks opening compared with pearlite,and that the cracks in ferrite-bainite steel were “locked” in the second hard bainite phase while the crack propagation path in ferrite-pearlite steel was more tortuous.Moreover,the fatigue fracture surface analysis and the coincidence site lattice(CSL) obtained by EBSD indicated that low-CSL grain boundaries in ferrite-bainite steel distributed more uniformly,which has a more significant effect on the resistance of crack propagation.It was revealed that ferrite-bainite dual-phase microstructures could inhibit the fatigue crack propagation more effectively than ferrite-pearlite microstructures. The purpose of this study was to describe the roles of microstructure types and grain boundary characteristics in fatigue crack propagation behavior in ferrite-pearlite steel and ferrite-bainite steel. The ferrite-bainite dual-phase steel was obtained by intermediate heat treatment conducted on ferrite -pearlite low carbon steel.This paper presents the results from investigation using constant stress-controlled fatigue tests with in-situ scanning electron microscopy (SEM), electron backscattering diffraction (EBSD) and fatigue fractography analysis. Microsroscopic images arrested by in-situ SEM showed that the second hard bainite phase distributed in the soft ferrite matrix had a significant effect on preventing the cracks opening compared with pearlite, and that the cracks in ferrite-bainite steel were “locked” in the second hard bainite phase while the crack propagation path in ferrite-pearlite steel was more tortuous. Moreover, the fatigue fracture surface analysis and the coincidence site la ttice (CSL) obtained by EBSD indicated that low-CSL grain boundaries in ferrite-bainite steel distributed more uniformly, which has a more significant effect on the resistance of crack propagation. It has been revealed that ferrite-bainite dual-phase microstructures could inhibit the fatigue crack propagation more effectively than ferrite-pearlite microstructures.