Because of an excellent combination of strength and ductility, mono-phase lowalloyed steels with a bimodal grain size are an appropriate alternative to conventionally cold-rolled and annealed steels as well as to steels with a dual-phase microstructure. This study investigates how the microstructure of a low-alloyed air-hardening steel either with a homogeneous, a dual-phase, or a bimodal grain structure influences its mechanical and fatigue performances. The homogeneous ferritic grain microstructure of the steel sheets is adjusted by an intercritical annealing at 790 C, along with subsequent air hardening to obtain a dual-phase state. Then, the ferritic-martensitic material is cold-rolled and annealed at 550-700 C to produce different bimodal grain microstructures. The evolution of microstructure and mechanical properties are characterized. An annealing temperature of 600 C is considered to be the optimal temperature resulting in pronounced bimodal grain size distribution. The sheet with a bimodal microstructure exhibits a higher strength and equal ductility compared to one with a homogeneous ferritic microstructure. Additionally, high-cycle fatigue tests of the material with a bimodal microstructure shows its superior fatigue behavior at a loading of above 800 000 cycles compared with both the homogeneous ferritic microstructure as well as the dual-phase microstructure.