Abstract: IF steels with average grain size of 40 μm and 210 μm were prepared by annealing at 850 ℃ and 950 ℃, respectively. The effects of the grain size on the low-cycle fatigue behavior and the microstructure after fatigue were investigated. The results show that during fatigue cycle, the initial average peak stress of the fine-grained test steel was higher than that of the coarse-grained test steel. The average peak stress of the two test steels tended to be equal with increasing number of cycles. The fine-grained test steel always showed cyclic work hardening during fatigue, while the coarse-grained test steel showed initial cycle hardening, cycle saturation and secondary cycle hardening. After 400 fatigue cycles, the microstructure of the fine-grained test steel was composed of similarly sized and uniformly distributed dislocation cells, while the microstructure of the coarse-grained test steel was mainly composed of macro-persistent slip band (Macro-PSB) and dislocation cells. The Macro-PSB contained relatively small dislocation cells. The coarse-grained test steel had higher dislocation density and relatively significant structural heterogeneity.