A relationship between the surface tension of cast iron and the graphite shape has been proven by experimental analysis and thus the graphite shape can be forecast by the surface tension. A specific relation between the surface tension and the graphite shape has been established by experiments. When the surface tension is above 1385 mN/m, nodular graphite occurs. When the surface tension is between 1283 mN/m and 1385 mN/m, vermicular and nodular graphite occurs. When the surface tension is between 1108 mN/m and 1283 mN/m, vermicular graphite occurs. When the surface tension is between 990 mN/m and 1108 mN/m, flake and vermicular graphite occurs. When the surface tension is below 990 mN/m, flake graphite occurs.