The experiment employed a co-precipitation method combined with conventional sintering to prepare 15 mol% yttria-doped ceria (YDC15) ceramics with various grain sizes (66 nm to 1.15 μm). All samples exhibit a single fluorite-type structure, with relative densities ranging from 93% to 97%. The grain conductivity of YDC15 decreases significantly as the grain size increases from 66 nm to 0.19 μm, and gradually increases as the grain size further grows to 1.15 μm. Both the apparent and specific grain-boundary conductivities of YDC15 show an overall declining trend with grain growth. The most pronounced decrease occurs during the transition from the nano- to the submicron-scale grains. The total conductivity of YDC15 follows the same trend as the apparent grain-boundary conductivity, exhibiting non-monotonic behavior. This synchronous change indicates that the grain-boundary resistance plays a dominant role in the total conductivity of YDC15. Among all samples, the nanocrystalline samples (66 nm) exhibits the optimal performance in the high-temperature range (500~650 ℃), with conductivity values from 2.46×10^-3 to 1.51×10^-2 S/cm. The submicron-grained sample (0.51 μm) show the best performance in the low-temperature range (200~450 ℃), with conductivities ranging from 1.54×10^-6 to 1.63×10^-3 S/cm. The influence of grain size on oxygen ions transport in the grain and at the grain boundary in YDC15 is not monotonic.