In this study the mechanical properties of ferritic ductile cast iron in as-cast are discussed by metallographic considering the effect of phosphorus content(about 0-0.8wt.%). In ferritic S.G. cast iron containing about 4.2wt.% Si, 0.5wt.% Ni, 0.02wt.% B, these samples are investigated for castability, microstructure, machinability, wear resistance, mechanical and thermal properties. The main results are summarized as follows. 1) The chill depth increased greatly with P addition up to about 0.6wt.% but the tensile strength and the elongation are decreased smoothly. 2) The size of S.G, and the amount of steadite increase with increasing P content. This means the tendency of increasing the number of S.G. 3) The maximum value of wear by mechanical fracture was appeared at the abrasion speed of 1.14m /sec and the plastic Flow layers are stretched longly on the wear surface with decreasing P content. 4) Although the coefficient of thermal expansion increased with P addition, the cast iron growth was slowing down extremely at 0.6w.% P or more.