As a result of the ferroelectric phase transition, piezoelectric ceramics develop complex internal stresses, which affect both the strength and electrical properties (dielectric constant and aging). However, very little quantitative information on these stresses is presently available. Furthermore, the effect of microstructure (e.g. grains, domains and pores), flaw sizes and internal stress on strength has not been understood. Thus, this paper presents a study of the mechanical properties of BaTiO3 showing that the strength of ∼99% dense BaTiO3 with typical machining flaws is 6.28 × 107 N/m2 higher at 150°C, which is above the Curie temperature of 120°C, than at 25°C. This measured difference in strength was relatively independent of grain size. Analysis of measured flaw sizes, elastic constants, and fracture energies show that the measured difference in strength is due to internal stresses. Studies of specimens with flaws of controlled size, introduced by hardness indenting at variable load, show that the difference in strength between 150°C and 25°C decreases as flaw size increases. Commercial sintered BaTiO3 had smaller differences in strength between 150°C and 25°C, which is attributed to the larger flaw sizes of the commercial materials, with porosity also being a possible factor. Several models are used to estimate these internal stresses, giving values generally consistent with experimental data.