Silver nanoparticles were surface functionalized with diacids and then incorporated into the polymer matrix with silver flakes as conductive fillers. By using appropriate diacids, the resistivity of the silver flakes and nanoparticles (molar ratio of silver flakes to nanoparticles is equal to 6:4) incorporated polymer composites was dramatically reduced to as low as 5 × 10-6 Ω·cm. Morphology studies showed that the decreased resistivity resulted from the sintering of silver nanoparticles. The contact resistance of the polymer composites under 85 °C and 85% relative humidity was significantly stable with respect to aging time as a result of the further sintering of silver nanoparticles.