Ca3Co4-xZnxOy (x = 0.01, 0.03, and 0.05) polycrystalline thermoelectric ceramics have been prepared by the classical solid state method. XRD data have shown that Ca3Co4O9 is the major phase, with small amounts of the Ca3Co2O6 one. Moreover, it has been found that the Zn has been incorporated into these two phases. Slight Zn doping decreases electrical resistivity compared with the values obtained in undoped samples. The minimum values have been obtained for the 0.01-Ni doped samples, increasing for further Zn substitution. Seebeck coefficient does not appreciably change in all the measured temperature range, independently of Zn content. The improvement in electrical resistivity leads to higher power factor values for the 0.01 Zn-doped samples (about 30 %) than for the undoped ones. The maximum power factor at 800 A degrees C, around 0.27 mW/K-2 m is significantly higher than the best results obtained in Zn doped samples reported in the literature.