Three key points are discussed to manufacture 300Mpa cylinder block castings. The first one is the design of grey iron chemical composition. The chemical composition used for production is as follows: C: 3.20-3.30%, Si: 1.60-1.80%, Mn: 0.70-1.00%, S: <0.10%, P: <0.06%, Cu: 0.70-0.80%, Cr: 0.25-0.35%, Ni: 0.30-0.40%, Sn: 0.06-0.08%. The stream inoculation is adopted. Another key point is to increase the chilling effect of molding material. A kind of ceramic sand is used in the experiment. The ceramic sand is mainly composed of electric fusion bauxite. The material gives much stronger chilling effect. The tensile strength can be 20 MPa higher compared with ordinary silica sand. The third key point is to use the technology of synthetic cast iron. The 300 MPa cylinder block is successfully developed.