A new Metal Matrix Composite (hereafter MMC) material for cylinder block bores has been developed, which makes it possible to reduce engine weight and size. To decide type, size and volume fractions of the MMC reinforcements, wear and scuffing characteristics as well as tensile strength were evaluated. Based on wear test results, crystallized alumina-silica fiber and mullite particles were selected as MMC reinforcements. In the case of counter-part (piston ring material) wear, the smaller particle size, the lesser ring material wear. For scuffing characteristics, the smaller particle size and the higher volume fraction, the higher the scuffing resistance. However, it is necessary to limit the volume fraction of reinforcements and particle size, because too high volume fraction and too small particle size causes a degradation in the tensile strength of the MMC. This is caused by an increase in micro porosity of MMC. Therefore, MMC reinforced crystallized alumina-silica fiber (Vf5%) and 12 mullite particle (Vf10%) are selected as optimal material, because this MMC material supported the desired wear characteristic and tensile strength. Electrochemical machining (ECM) was adopted to ensure the scuffing resistance. Relationships between scuffing characteristics and ECM-processed bore surface characteristics were clarified. This MMC cylinder block was commercialized in August 1999.