Materials used for microelectronic devices need to have more multifunctional properties, such as excellent mechanical, thermal, and dielectric properties at the same time. In this paper, copper phthalocyanine/polyimide (CuPc/PI) composite films with high dielectric permittivity (k=9.9 with the volume fraction of CuPc is 28% at 100Hz) and high thermal stability are prepared by an in-situ polymerization process. The composite films show good dielectric properties and is almost independent of frequency in the measured frequency range up to 104 Hz. Notablely, The composites remain stable until the temperature reaches 500oC. In addition, the inclusion of CuPc slow down the composite degradation rate when the temperature is higher than 550oC. The resultant high performance of such polymer composites makes them attractive for technological applications in flexible high-k components.