Tensile strength of the SiC/Ti-6Al-4V composites under the conditions of preparation and heat treatment at 900 oC for different time was measured and calculated by means of Global Load-Sharing model. It is found that after long-time heat treatment, the calculated strength is in accordance with the measured one. But the former is higher than the later for the as-prepared sample. SEM and TEM observations show that with prolonging the holding time at 900 oC, the interfacial reaction zone of the SiC/Ti-6Al-4V becomes thicker and the C-coating of the SiC fiber thinner, leading to the decrease of the tensile strength and the increase of the interfacial shear strength. It is identified that the main interfacial reaction product is TiC. However, when the C-coating is consumed entirely Ti5Si3 forms in the interfacial reaction zone adjacent to the SiC fiber. Interfacial reaction is the main reason for the decrease of mechanical properties of composites.