The mechanical properties of ER8 steel for wheel rim were studied at -40℃, -20℃, 0℃ and 25℃ (room temperature) respectively, while the microstructure and fractured surface of the steel were characterized by means of laser confocal microscopy, field emission scanning electron microscopy (SEM). Results show that the tensile- and yield-strength increase linearly with the decreasing temperature, i.e. the increments of which reach 5.8% and 7.1% respectively at -40℃, in comparison to those at room temperature, correspondingly the plasticity index (elongation and cross section shrinkage) decreases by about 2%; The impact toughness of the steel for wheel rim is very sensitive to temperature, the impact toughness of the steel reduces rapidly with the decreasing temperature, and the impact energy reduces by 60% at -40℃ in comparison to that at ambient temperature; The fatigue life of the steel at -40 is higher than that at room temperature. At -40℃, the size of secondary cracks in the fatigue source and crack propagation zone is smaller than those at room temperature, the fatigue crack critical size a_c is about 3.2 mm at room temperature, while it is about 4 mm at -40℃。