The effects of NaNbO3 particle size on the photocatalytic activity were investigated by taking gaseous 2-propanol photodegradation as a model reaction. By varying the loading amount of precursors onto mesoporous SBA-15, nanocrystalline NaNbO3 samples with particle sizes ranging from 3 to 50 nm were successfully synthesized. The particle size and surface area value of templated NaNbO3 change systematically with the change in loading amount of the precursors. The physicochemical characteristics of the samples were examined by techniques such as XRD, SEM, TEM, Raman spectroscopy, and UV-visible diffuse reflectance spectroscopy. The optimum NaNbO3 particle size for the highest photocatalytic activity was found to be 30 nm. The current study also revealed that the NaNbO3 photocatalyst is structure sensitive for 2-propanol photodegradation. The density of active sites on the sample surface decreases as the particle size decreases. Competing effects of factors such as the surface area, changes in electronic structure, defect density, and concentration of reaction active sites determine the optimum particle size of NaNbO3.