Crystalline cubic cerium oxide nano particles have been synthesized from cerium (III) nitrate (Ce (NO₃)₃.6H₂O) and sodium hydroxide by a hydrothermal method. The effect of three different molar ratios of the NaOH precipitating agent on structural, optical, and photo catalytic activity was investigated. The synthesized cerium oxide nano particles were characterized by X-ray diffraction (XRD), a UV–vis spectrometer, scanning electron microscope (SEM), energy-dispersive X-ray spectroscopy (EDAX), Raman spectroscopy and X-ray photo electron spectroscopy (XPS). According to the findings, hydrothermally synthesized cerium oxide NPs have a high efficiency for photocatalytic degradation of methylene blue when exposed to UV light. Environmental water pollution is the major issue of the atmosphere. To get fresh water, humans could search the resources to purify the water in simple way and degradation is the one of the methods to purify salt water.