In this article, we report the use of a microwave plasma in a microwave plasma–assisted spray (MPAS) technique to grow crystalline nanoparticles of the oxide thermoelectric material Ca3Co4O9. This unique growth process allows the formation of nanoparticle coatings on substrates from an aqueous precursor of Ca and Co salts. The particle size is controlled from few tens to few hundred nanometers by varying the concentration of the precursor. The resistivity, Seebeck coefficient, and the power factor (PF) measured in the temperature range of 300–700 K for films grown by MPAS process with varying concentrations of calcium and cobalt chlorides are presented. Films with larger nanoparticles showed a trend toward higher PFs than those with smaller nanoparticles. Films with PFs as high as 220 μW/mK2 were observed to contain larger nanoparticles.