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In polycrystalline ceramics, charge trapping property has important impact on the dielectric breakdown strength (DBS). In this work, alumina ceramics with Ca 9 Al(PO 4) 7 (CAP) as the second crystalline phase were synthesized using coarse alumina as main raw materials, and the CAP crystal being dendritic or granular depends on the mass ratio of calcium phosphate to kaolin in the raw material composition. Microstructure and impedance spectrum indicated that dendritic CAP with typical fractal features greatly increases the charge trapping sites and charge spreading barrier. The AC DBS of ceramics with 70 wt% alumina and fractal dendritic CAP achieved a competitive value of 16.1 kV/mm. The results indicate that growing fractal dendritic CAP between the main crystalline grains is a very feasible way to enhance the DBS of alumina ceramic. • Dendritic fractal Ca 9 Al(PO 4) 7 can be obtained simply and stably. • Dendritic Ca 9 Al(PO 4) 7 greatly increases the charge-trapping property. • The effects of the morphology and content of the second phase on the breakdown dielectric strength were investigated. • An effective and low-cost way was provided to enhance the dielectric breakdown strength of alumina ceramics. [ABSTRACT FROM AUTHOR]