Abstract: The sedimentation of micron Al₂O₃ fillers can reduce the electrical and mechanical properties of epoxy composites and restrict the improvement of large size insulating parts. We carried out the research of 3-aminopropyltriethoxysilane (APTES) inhibiting the sedimentation of micron Al₂O₃ fillers in epoxy composites. The density method was used to measure the sedimentation characteristics of micron Al₂O₃ fillers in epoxy composites with different APTES amount. The effect of APTES amount on the electrical and mechanical properties of epoxy composites was studied. The research results show that the addition of APTES can reduce the sedimentation rate of micron Al₂O₃. With the increase of the addition of APTES, the density difference of the sample due to sedimentation first decreases and then increases, its electrical and mechanical properties first increase and then decrease, and the dispersion of properties first decreases and then increases. When the amount of APTES added is 1.5%m(Al₂O₃), the sedimentation of micron Al₂O₃ fillers is the smallest, the tensile strength, bending strength, breakdown strength and flashover voltage of the sample are the largest. It is hoped that the research results will provide references for the formulation development of epoxy castables and the manufacture of epoxy castables.