Optically transparent α-Al2O3 thin films were prepared on Si(100) substrates by electron cyclotron resonance (ECR) plasma-assisted pulsed laser deposition followed by heat annealing. Oxygen plasma produced through ECR microwave discharge was used to assist reactive deposition of amorphous aluminum oxide thin films from metallic aluminum and the deposited films were then annealed in air at temperatures ranging from 500 to 1100 °C. The as-deposited and heat-annealed films were characterized by Fourier transform infrared spectroscopy, Raman spectroscopy, and X-ray diffraction analysis. The as-deposited films exhibit an amorphous structure, undergo a phase transition upon heat annealing, and convert to α form of Al2O3 with rhombohedral crystalline structure after annealing at 1100 °C. A SiO2 layer is also found to form between the aluminum oxide film and the Si substrate after the samples were annealed above 700 °C. Optical characterization reveals that aluminum oxide films deposited on sapphire substrates under the same deposition conditions are transparent from ultraviolet to near-infrared regions, and the transparency increases over 10% for the α-Al2O3 films crystallized through annealing at 1100 °C as compared with that of the as-deposited films.