Abstract: Nano-SiC/Polyimide (n-SiC/PI) composite films were prepared by using in-situ dispersive polymerization. The surface morphology, thermal expansion, dielectric properties and thermal stability of n-SiC/PI were studied by SEM, thermal mechanical analysis (TMA), impedance analyzer and thermal gravimetric analysis (TG) respectively. The results show that n-SiC particles are dispersed in the PI matrix evenly by employing the in-situ polymerization. The coefficient of thermal expansion (CTE) of n-SiC/PI composite films decreases with the increasing of the SiC content, while the experimental data could be analyzed by Kerner model closely. The CTE of PI with n-SiC mass fraction of 15% decrease about 11% than that of the pure PI. The dielectric constant and dielectric loss of films vary with the content of n-SiC fillers, remaining in the lower range and stable in a wide frequency range.