One of the main hurdles for the wide use of current Ag-filled conductive composites is the high cost of Ag fillers, while the challenge for low cost copper-filled composites is their poor reliability. In this study, the Ag-coated Cu flakes was chosen as conductive fillers and a surface modification with coupling agent, to achieve good bonding between the flakes and the Polydimethylsiloxane (PDMS) matrix, was applied to improve the mechanical and conductive properties of the PDMS based composites. The resulting composites obtained through modification showed an increase in hardness by 13.1% and tensile strength by 28.4% relative to the composites without coupling agent. In addition, the reliability of conducting composites was discussed and the results showed that the coupling agent played a great role in preventing the corrosion of the exposed Cu during aging. The volume resistivity of the composites, filled with untreated Ag-coated Cu flakes, increased from 0.0039Ω·cm to 0.0059Ω·cm under aging at 85°C and 85% room humidity (RH) in a temperature/humidity chamber for 48 h, in contrary, the volume resistivity shift of the composites with treated flakes was almost negligible, just changed from 0.0043Ω·cm to 0.0045Ω·cm. More importantly, the prepared conductive silicone rubber filled with Ag-coated Cu flakes with the resistivity (0.004Ω·cm) comparable to the relative commercial products can be used for electro-magnetic interference (EMI).