Abstract: The effects of rear earth,manganese and silicon elements on ferrite hardness of nodular cast iron used for crank shaft were analyzed.Samples of ferrite nodular iron crank shaft were fabricated.Mechanical properties of samples were measured and microstructures of materials were observed.Silicon was the main element which dissolved into ferrite to form substitutional solid solution,and caused lattice distortion and increased hardness.Within the tested range,the silicon content of the ferrite nodular iron was linear with hardness.During production,the hardness range of the crank shaft might be controlled through adjusting the silicon contents of the molten iron.Thus the demand of machinability was satisfied.