In this work, TiO₂ nanoparticles were successfully synthesized with narrow size distribution via a wet ball milling sol-gel method. The effect of calcination temperature on photocatalytic activity was observed from particle size, crystallite size, and phase transition of TiO₂ nanoparticles. Increasing calcination temperature increased particle size, crystallite size, and the crystallinity of synthesized TiO₂. Phase transition depended on variation in calcination temperatures. A two-phase mixture of anatase and brookite was obtained with lower calcination temperature whereas a three-phase mixture appeared when calcination temperature was 500–600 °C. With higher temperature, the rutile phase kept increasing until it was the only phase observed at 800 °C. Anatase strongly affected the photocatalytic activity from 300 °C to 600 °C while the particle size of TiO₂ was found to have a dominant effect on the photocatalytic activity between 600 °C and 700 °C. A mixture of three phases of TiO₂-600 exhibited the highest methylene blue degradation with the rate constant of 9.46 × 10⁻² h⁻¹ under ultraviolet (UV) irradiation.