A study is presented comparing the statistics of observed crack-originating flaws on fracture surfaces of samples of Al2O3 and its composites (having 5 and 15 vol % SiC particle, SiCp) with those determined by the Hertzian indentation analysis. Cracks originating from Vickers microindents are also examined using transmission electron microscopy (TEM). Critical flaws determined by the Hertzian analysis predict experimental fracture strengths reasonably well when Orowan＇s model of obtaining the latter is used. This model is supported by microstructural features, which strongly suggest blunt crack-tips. The critical flaw sizes of alumina and the 5 vol % SiCp composite are found to be practically the same. Hence the explanation of the higher strength of the composites (relative to alumina) based on flaw size is improbable. The composite with 15 vol % SiCp is tougher and has a lower fracture strength in comparison with those of the 5 vol % SiCp composite. Therefore an increase in toughness does not satisfactorily explain the strengthening trend. The change in the mode of fracture and the interspacing distances (implicitly the heights) of the fracture steps are suggested as the main factors that control strengthening in these materials.