In this paper, we investigated the effect of nonconducting fillers on the thermomechanical properties of modified anisotropic conductive adhesive (ACA) composite materials and the reliability of flip chip assembly on organic substrates using modified ACA composite materials. For the characterization of modified ACAs composites with different content of nonconducting fillers, dynamic scanning calorimeter (DSC), and thermogravimetric analyzer (TGA), dynamic mechanical analyzer (DMA), and thermomechanical analyzer (TMA) were utilized. As the nonconducting filler content increased, CTE values decreased and storage modulus at room temperature increased. Tn addition, the increase in the content of filler brought about the increase of Tg(DSC) and Tg(TMA). However, the TGA behaviors stayed almost the same. Contact resistance changes were measured during reliability tests such as thermal cycling, high humidity and temperature, and high temperature at dry condition, The reliability results were significantly influenced by CTEs of ACA materials, especially at the thermal cycling tests. Results showed that flip chip assembly using modified ACA composites with lower coefficients of thermal expansion (CTEs) and higher modulus by loading nonconducting fillers exhibited better contact resistance behavior than conventional ACAs without nonconducting fillers.