A boron-doped alumina (Al2O3:B) dosimeter was developed for thermal neutron dosimetry. The Al2O3:B was synthesized by a sol-gel method, and annealed up to 1200°C. The ratio of B to Al in the Al2O3:B was 0.1. For X-ray diffraction (XRD) analysis, the diffraction pattern of the Al2O3:B at 500°C had broad reflex peaks corresponding to the crystalline structure of γ-Al2O3. The XRD pattern at 1200°C had reflex peaks corresponding to the crystalline structures of α-Al2O3 and Al18B4O33. The optically stimulated luminescence (OSL) property of the Al2O3:B had satisfactory linearity in X-ray dose measurement. Several type dosimeters, Al2O3, Al2O3:B, and Al2O3:B with Cd foils, were set in a thermal neutron field. It was confirmed from their responses that the response of Al2O3:B was originated from 10B(n,α) 7Li reactions.