GDC20($Ce_{0.8}Gd_{0.2}O_{1.9}$) powder was synthesized by oxalate co-precipitation and milling and its thermal decomposition, phase formation, and sinterability were investigated. As-prepared precipitates were non-crystalline due to the milling process and completely decomposed at 400$^{circ}C$ The powder calcined at 800$^{circ}C$ for 2 h contained fine p]sty particles with an average size of 0.69 $mu$m. Attrition milling of the calcined powder for 2 h had a little milling effect, resulting in a slight decrease in the particle size to 0.45 $mu$m. The milled powder consisted of small spherical primary particles and some large particles, which had been agglomerated during calcination. Due to the excellent sinterability of the powder, sintering of the powder compacts for 4 h showed relative densities of 78.7% at 1000$^{circ}C$ and 97.8% at 1300$^{circ}C$, respectively. Densification was found to almost complete at temperature above 1200$^{circ}C$ and a dense and homogeneous microstructure was obtained. A rapid grain growth occurred between 1200$^{circ}C$ and 1300$^{circ}C$. Grains in 0.1$sim$0.5 $mu$m sizes at 1200$^{circ}C$ grew to 0.2$sim$2 $mu$m and their size distribution became broader at 1300$^{circ}C$.