Ca3Co4O9 samples synthesized from micrometer (CaCO3 and Co3O4) and nanometer (CaO and Co3O4) sized precursors have been prepared by solid state reaction, sintered by spark plasma sintering. Their thermoelectric properties at high temperature have been studied (650-1000 K). Using one nano-sized precursor seem to allow synthesis of Ca3Co4O9 particles with smaller average size (<1 mu m) whereas using two nano-sized precursors seems to lead to two different Ca3Co4O9 particles populations, one close to 1 mu m and a smaller one near 200 nm. The nature of the precursor does not seem to significantly affect the Seebeck coefficient. However, when using a nano-sized precursor a slightly higher electrical conductivity in the whole temperature range is observed. This leads to slightly higher power factors for samples prepared from nano-sized precursors. The main effect is observed on thermal conductivity that is always lower for samples synthesized from nano-sized precursors (from 2.10 to 1.65 W m(-1) K-1 at 1000 K for CaCO3-Co3O4 and nanoCaO-Co3O4 samples respectively). This thermal conductivity reduction due to smaller average Ca3Co4O9 particles size is consistent with previously published models. The best ZT value reaches 0.21 at 1000 K for the sample synthesized from nano-sized CaO and micrometer sized Co3O4. (C) 2012 Elsevier Ltd. All rights reserved.