The crankshafts of semi-finished products were manufactured by two processes: as-cast followed by normalizing and iron mould with sand liner as-cast. Then those were machined and nitridized followed by the bending fatigue test. At last mechanical properties and impact properties of sampled crankshafts were tested. The mechanical properties are similar in the crankshafts treated by both of the processes, however, the bending fatigue limits of as-cast spherical graphite iron crankshafts are much higher than that of normalized spherical graphite iron crankshafts, whose crack producing work and extending work is higher as well. The higher extended work is considered to relate to the oeil-de-boeuf ferrite and graphite structure and pearlite in as-cast spherical graphite iron crankshafts, while the normalized ones with lower fatigue limit value are composed of spherical graphite and pearlite. Cracks having initiated in the graphite, the tips of cracks can be blunted and the crack propagation can be impeded efficiently by ferrites surrounding the graphite in this structure.