Abstract: Yellowish and transparent rutile single crystal (ø20 mm × 30 mm) doped with 0.4% (atom fractions) Al2O3 was successfully produced by the flame fusion (improved Verneuil) method, whereas a pure rutile crystal without the doping is dark-blue and not transparent. It is shown that the substitution of Al³⁺ for Ti⁴⁺ reduces the number of Ti³⁺ due to the reduction of Ti⁴⁺, and reduces the non-natural oxygen vacancies and the amount of oxygen needed in the crystal growth. Oxygen diffusivity in the crystal is enhanced by the doping because the radius of A1³⁺ is less than that of Ti⁴⁺ and oxygen diffusion path can be widened. Natural 2Al′Ti-V¨O defect pairs form in the Al-doped rutile to provide paths for the migration of oxygen ions. The introduction of Al into the crystal speeds up the diffusion of oxygen ions and eliminates the formation of substantive color centers.