We synthesized TiO2 mesocrystals using a hydrothermal method and investigated the effect of calcination temperature (100-800 degrees C) on their morphology, crystallinity, and photocatalytic activity. While no appreciable changes in the shape, dimension, and crystal structure of the TiO2 nanoparticles (NPs) were observed as the calcination temperature increased to 300 degrees C, the crystallinity improved with increasing temperature. The mesocrystal form of the NPs began to disappear at 400 degrees C, and the specific surface area significantly decreased with increasing temperature owing to the reduced boundaries between the subunits and surface toughness of the NPs. The photocatalytic activity of the TiO2 NPs improved when the temperature increased to 300 degrees C because of the enhanced crystallinity and elimination of byproducts; on the other hand, it degraded above 400 degrees C due to the decreased surface area. These results suggest that controlling the calcination temperature is an effective way to tailor the morphology, crystallinity, and photocatalytic activity of TiO2 NPs.