Epoxy ternary blends (DCN) were prepared by mixing diglycidyl ether bisphenol A (DGEBA), cycloaliphatic epoxy, and novolac epoxy. The silica diffusants were prepared by the addition of spherical silica (SS) into epoxy blends. The thermal properties of the epoxy composites were characterised using a thermo-mechanical analyser (TMA), a differential scanning calorimeter (DSC), and a dynamic mechanical analyser (DMA). It was found that the storage modulus of the epoxy was increased in the presence of SS diffusants. However, the coefficient of thermal expansion (CTE) and the glass transition temperature (Tg) of the epoxy ternary blends was reduced by the addition of SS diffusants, which was because the expansion of the epoxy matrix was constrained in the presence of silica fillers. The UV/Vis spectroscopy results demonstrated that the percentage of transmittance of epoxy was decreased by the incorporation of the silica diffusant.