Ceria is an important rare-earth oxide with ever increasing applications. In nanocrystalline form, ceria exhibits novel and improved properties compared to its microcrystalline counterpart. The variation in the properties was observed to be a function of lowering particle size. This was also attributable to the presence and extent of Ce in 3+ state in ceria. Thermal decomposition of a thermally less stable cerium metal salt is an useful and simple method for the synthesis of nanocrystalline ceria. As atmosphere under which the decomposition occurs is expected to alter the Ce3+ content and thus the property, the present study is to investigate the effect of reactive atmosphere of decomposition vis-à-vis the particle size on the property of nanocrystalline ceria. Nanocrystalline ceria was synthesized by controlled thermal decomposition of cerium nitrate under four different atmospheres and the products were analyzed by X-Ray Diffraction, X-ray photoelectron spectroscopy and Laser Raman Spectroscopy. The observed property variation was found to be more dependent on the Ce3+ content thus the synthesis atmosphere rather than the particle size.