Abstract: This paper deals with high-tensile-strength C/Mg composite with b>1000M Pa fabricated by vacuum casting.T 300 fibers coated with C-Si-O gradient coatings are aligned in casting moulds,and then infiltrated with molten ZM-5 alloy under vacuum 4000Pa.The tensile strength of specimens containing fibers of 0.35 volume fraction reaches up to 1050M Pa.The effects of C-Si-O gradient coating on tensile strength of composites are mainly discussed.