-In this study, AC breakdown properties of cenosphere/epoxy composite are investigated.The thermal properties of the composite are also discussed. The cenosphere/epoxy composites were prepared by epoxy resin filled with 0~40 wt% of hollow fly ash particles (cenospheres). The AC breakdown strength of the composites was measured at 25~125℃. The permittivity, glass transition temperature, and coefficient of thermal linear expansion of the composites were also measured. The experimental results of the cenosphere/epoxy composite were compared with those of the silica/epoxy composite which are widely used as insulating materials in power apparatus. The cenosphere/epoxy composite showed the similar or better performance than silica/epoxy composite in terms of breakdown strength, glass transition temperature, and coefficient of thermal expansion.