New high-performance photocatalytic materials are required to effectively treat water pollution. The effect of annealing temperature on the photocatalytic activities of tin dioxide (SnO₂) nanoparticles is investigated in this work. SnO₂ nanoparticles are prepared via a hydrothermal method and annealing at various temperatures ranging from as-prepared to 900 °C. The size of SnO₂ nanoparticles increases from 4 nm to 10 nm with an increase in annealing temperature. The photocatalytic properties of these nanoparticles are evaluated through the photocatalytic degradation of methylene blue under sunlight. Photocatalytic activities decrease significantly with an increase in annealing temperature due to an increase in size and a decrease in the surface area of SnO₂ nanoparticles.