In this paper, the unidirectional SiC fiber-reinforced aluminum phosphates composites, in which the SiC fibers were heat-treated at different temperature, time and in different method, were prepared. The dielectric and mechanical properties of the composites were studied. The influences of heat treatment of SiC fiber on the properties of the SiC fiber-reinforced aluminum phosphates composite were investigated in detail. The flexural strength, relative dielectric constant and dielectric dissipation factor of the composite were measured. And the microstructure of the composite was characterized by SEM (scanning electronic microscope). The results show that heat treatment of SiC fiber has a great influence on mechanical and dielectric properties of the composite. The heat treatment decreases the dielectric constant and dielectric dissipation factor of the composite enormously. But at the same time, the heat treatment of the SiC fiber makes an unfavorably strongly bonded SiC fiber/aluminum phosphates matrix interface, which decreases the strength of the composite extraordinarily. And the composite displays a completely brittle failure behavior without fiber debonding and pulling out, which is detected by SEM.