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By introducing ZrO2 (4Y) powder into the thermit, Al2O3/ZrO2 (4Y) composite ceramics were prepared through combustion synthesis under high gravity, and the correlations of composition, microstructures and mechanical properties of composite ceramics were investigated. The results of XRD, SEM and EDS showed that Al2O3/33%ZrO2 (4Y) were composed of random-orientated rod-shaped colonies consisting of a triangular dispersion of orderly submicron-nanometer t-ZrO2 fibers, surrounded by inter-colony regions consisting of spherically-shaped micronmeter t-ZrO2 grains; Al2O3/45%ZrO2 (4Y) were comprised of spherically-shaped micron-meter t-ZrO2 grains, surround by irregularly-shaped α-Al2O3 grains and a few colonies. Compared to the directionally solidified Al2O3/ZrO2 (Y2O3), the increase in hardness and flexural strength of Al2O3/33%ZrO2 (4Y) in the experiment was due to high densification, small-size defect and high fracture toughness induced by compressive residual stress toughening and transformation toughening mechanisms; meanwhile, in despite of the moderate decrease in hardness, high flexural strength of Al2O3/45%ZrO2 (4Y) was considered to be a result of small-size defect in spherically-shaped micronmeter t-ZrO2 grain matrix and high fracture toughness induced by transformation toughening and micro-crack toughening mechanisms.