Chemically durable microspheres 20−30 µm in diameter containing a large amount of yttrium are useful for in situ radiotherapy of cancer as they can be activated by neutron bombardment to become β-emitters and can be injected in the vicinity of the cancer to provide a large localized dose of β-radiation. In this study, preparation of hollow Y2O3 microspheres using an enzymatic reaction was attempted, and the structure and chemical durability of the resulting microspheres were investigated. Hollow Y2O3 microspheres 20–30 &m in diameter were successfully prepared by this enzymatic method. The outer surface of the microspheres was smooth and dense, whereas the inner parts had a honeycombed structure. In simulated body fluids at pH 6 and 7, the hollow Y2O3 microspheres showed high chemical durability.