Abstract Thermal expansion coefficients along three main crystallographic axes of Tm-doped YAlO 3 (Tm:YAP) single crystals were investigated in the temperature range of 30–1350 °C, and the results showed that they are 11.9458, 5.0012 and 10.5439 × 10 −6 K −1 , respectively. Polarized absorption and fluorescence spectra of Tm:YAP crystals in a – c directions were measured. The absorption cross sections near 794 nm are 8.64, 9.22 and 6.80 × 10 −21 cm 2 , respectively. According to the fluorescence spectra, the b -axis emission cross section at 1.86 is determined to be 4.33 × 10 −21 cm 2 , larger than those along two other axes. The results above showed that the anisotropy effect of thermal and optical characteristics exists in Tm:YAP crystal and b orientation is the preferred one. The origin and peculiarities of anisotropy were investigated and discussed.