The nanostructured cerium oxide powders were synthesized by spray thermal decomposition process for the use as the raw materials of resistive oxygen sensor. The synthesis routes consisted of 1) spray drying of water based organic solution made from cerium nitrate hydrate ($Ce(NO_3){_3}6H_2O$) and 2) heat treatment of spray dried precursor powders at $400^{circ}C$ in air atmosphere to remove the volatile components and identically to oxidize the cerium component. The produced powders have shown the loose structure agglomerated with extremely fine cerium oxide particles with about 15 nm and very high specific surface area ($110m^2/g$). The oxygen sensitivity, n ($Log{propto}Log (P_{O2}/P^o)^{-n}$ and the response time, $t_{90}$ measured at $600^{circ}C$ in the sample sintered at $1000^{circ}C$, were about 0.25 and 3 seconds, respectively, which had much higher performances than those known in micron or $100{sim}200nm$ sized sensors.