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Epoxy/microsilica (65 phr)/nanosilica (0~5 phr) composites (EMNC) were prepared in order to develop a high-voltage insulation material, where phr means parts per hundred relative to the epoxy oligomer. Tensile and flexural tests of the composites were carried out, and the AC electrical breakdown strength was measured, after which all the data were estimated by Weibull statistical analysis. As the nanosilica content increased, the tensile strength increased, and the highest value was 117.7 MPa in the EMNC system with 3 phr nanosilica, which was ca. 10% higher than that of the system without nanosilica. The value then decreased after 3 phr. The flexural strength and AC electrical breakdown strength showed the same tendencies as the tensile strength. The highest value of the flexural strength was 184.6 MPa in the EMNC system with 3 phr of nanosilica, which was ca. 15% higher than that of the system without nanosilica. The strongest value of the AC electrical breakdown strength was 79.0 kV/0.5 mm in the EMNC system with 3 phr of nanosilica, which was ca. 34% higher than that of the system without nanosilica.