Hydroxyapatite (HAp) is a bioactive ceramic material with a variety of current and potential applications in medical technology. HAp is noncubic and, thus, should be subject to microcracking due to thermal expansion anisotropy (TEA) if the grain size, G>G c r , the critical grain size for TEA-induced microcracking. However, no estimates of G c r are available in the literature. Although a range of thermal expansion values reported in the literature complicates the estimation of G c r for HAp, a G c r of 1.1 μm for polycrystalline HAp is estimated using data from the literature and from the authors' measurements.