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A four-point rotating bending fatigue test has been performed on amartensite-bainite low alloyed steel rod material. The influences of microstructure on thecrack initiation and propagation path have been investigated. Subsurface crack initiationwas observed in all the samples tested. Most of the crack initiation sites were not locatedat the internal defects such as inclusions or pores, but at the areas with sizes of 4 to 14grains in the microstructure (subsurface non-defect crack origin-SNDCO). The sizes ofboth crack initiation site and “fish eye” increase with decreasing applied stressamplitude. The SNDCO appearances have also changed from more ductile fracture tofacet with ridge. This phenomenon was explained using expanded Kitagawa diagrams.The subsurface crack initiation started either in the ferrite phase in bainite due to theintrusion and extrusion process or at grain boundaries due to the stress concentration bythe pile-up of dislocations. The transition from shear cracking to tensile cracking led tothe formation of crack initiation sites. A ”fish eye” type of fracture was followed beforethe final stage cracking. The influence of microstructure on the fatigue crack initiationand propagation lives was discussed.