Herein, dually doped Ca2.5Ag0.3RE0.2Co4O9 (RE: La, Pr, Nd, Sm, Gd, Dy, Er, Yb, Eu, Tb, Ho, Lu, Ce, and Y) samples are synthesized by sol-gel technique and consolidated by cold pressing under high pressure to systematically scrutinize the influences of Y and rare-earth dually doping with Ag on transport properties of Ca3Co4O9 for high-temperature thermoelectric (TE) applications. Characterization results reveal that targeted phase is successfully produced, and doping of the compositions is provided. Doping of Y and rare-earth elements together with Ag into the Ca2+ site is effective in increasing the Seebeck coefficient and decreasing the electrical resistivity of the samples, thanks to the reduction in carrier concentration. Thermal conductivity of the samples is reduced related to the lower relative densities and alloy scattering originated from dually doping. Among the samples, Ca2.5Ag0.3Ho0.2Co4O9 and Ca2.5Ag0.3Eu0.2Co4O9 exhibit the highest power factor (PF) values of 0.65 and 0.62 mW m(-1) K-2 at 800 degrees C, respectively. These results are quite high for bulk oxide TE materials which can be assessed as potential oxide TE materials for high-temperature TE power generation.