This paper presents an attempt to derive the dc dielectric breakdown characteristic of epoxy composite containing alumina particles with mesoporous structure (Mesoporous-alumina/epoxy composite). By compounding epoxy resin and mesoporous-alumina particles which had the micrometric particle-diameter and the nanometric pore-size, mesoporous-alumina/epoxy composite was fabricated. The measurement of the specific gravity of mesoporous-alumina/epoxy composites revealed that the porosity of mesoporous-alumina particle even in the epoxy matrix was higher than that of nonporous-alumina particle. Furthermore, the measurement of the dielectric permittivity of the mesoporous-alumina/epoxy composites verified the existence of pore inside the mesoporous-alumina particle from the view point of electrical characteristics. From the breakdown test with using Mckeown electrode system, DC breakdown strength of mesoporous-alumina/epoxy composites was obtained, which was compared with that of the nonporous-alumina/epoxy composites. These results suggested that the nanometric pore with wormhole structure inside mesoporous particle could not be the critical defect for DC dielectric breakdown characteristic.