Abstract: In order to prepare the nanoparticles filling silicone composites with favorable thermal conductivity, thermal stability, electrical conductivity and flexibility, vinyl terminated polydimethylsiloxane (PDMS) was used as matrix material and carbon coated cobalt nanoparticles (C@Co) was used as filler, C@Co/PDMS composite thermal interface materials were prepared by ground co-blend method firstly. Then, TEM, XRD, Raman, and SEM were employed to investigate the microstructure, phase, degree of graphitization and dispersibility of C@Co. Finally, the effects of C@Co content on thermal conductivity, thermal stability, electrical conductivity and flexibility of composite thermal interface materials were investigated. The results show that the thermal conductivity of the composite thermal interface materials increases with the C@Co content increasing, when the C@Co content is 24wt%, the thermal conductivity of the composite reaches the maximum 1.64 W/(m·K), which is 10.7 times higher than that of neat PDMS. TG analysis indicates that after the addition of 24wt% C@Co, the initial decomposition temperature and final decomposition temperature of composites increase about 70 ℃ and 80 ℃ comparing to that of the neat PDMS respectively, accounting for C@Co can enhance the thermal stability of the composites. With the C@Co content increasing, the electrical conductivity of the composite thermal interface materials increases nonlinearly, and the percolation threshold value of fitting trial and error calculation is 10wt%, which means that when the C@Co content is less than 10wt%, the insulativity of composite is favorable; but when filled with 24wt% C@Co, the electrical conductivity of composite is 9.38×10^-3 S·m^-1. The hardness of composites is moderate, which is in the range of 17.6-26.8 HA and indicates that the flexibility of the composite is preferable. Therefore, 24wt% C@Co/PDMS composite can not only meet the basic requirements of the electrical properties of thermal interface materials, but also have favorable thermal conductivity, thermal stability and flexibility.