Bi2O3 nanostructures with high surface-to-volume ratio are attracting particular interest in catalysis, gas sensors, solar-energy conversion, and so on. In this paper, we present the impact of annealing on morphology, structure, and optical properties of Bi2O3 nanotube-based urchin-like microarchitectures, which is very important to determine before their applications in many potential fields. Our results showed that below 400 °C, the microarchitectures possessed good thermal stability. While annealing at 500 °C, jointed nanoparticles with an average size of 350 nm were formed. On further increasing the annealing temperature to 600 °C, the microarchitectures were transformed into stable α-Bi2O3 microcrystal film with a larger particle size of about 800 nm. The optical bandgaps of the annealed products were also changed according to their changes of morphology and structure.