Polymer composites with high thermal conductivity offer new possibilities for thermal management in electric systems. An approach to improve thermal conductivity of polymer is the addition of filler with high thermal conductivity. The purpose of this study is to investigate the thermal and mechanical properties of alumina/epoxy composites, and the effect of addition of reactive diluent (RD) to the epoxy matrix on the properties of alumina/epoxy composites is examined. Thermal conductivity of alumina/epoxy composites fabricated by different condition was measured by steady-state method. Additionally, flexural strength and failure strain were evaluated to examine the effect of addition of RD to the epoxy matrix on the mechanical properties of alumina/epoxy composites. Moreover, dynamic mechanical analysis was performed to evaluate the glass-transition temperature (Tg) of alumina/epoxy composites.