In this study, TiO2 nanotube (TNT) arrays were grown on Ti metal using the anodic oxidation method. After finding the parameters to grow TNT arrays with maximum specific surface area, the TNT arrays were annealed over a temperature range of 350–600 ℃. From the X-ray diffraction (XRD) patterns we found that the as-deposited TNT arrays revealed an amorphous phase. As the annealing temperature was increased from 350 to 500 ℃, the diffraction intensity of the anatase phase increased. As the annealing temperature reached 600 ℃, both the anatase phase and the rutile phase were observed in the arrays. The TNT arrays with different annealing temperatures were used to carry out experiments in moss decomposition and methylene blue degradation. The surface observation was used to judge the effect on moss decomposition, and the Beer–Lambert law was used to judge the effect on methylene blue degradation. We show that TNT arrays annealed at 500 ℃ functioned best in both moss decomposition and methylene blue degradation because of the presence of only the anatase phase.