Polycrystalline samples of [(Ca_1-xSr_x)2CoO₃]_ρCoO₂ with 0≤x≤0.20 have been synthesized at 1193 K in flowing oxygen gas. The modulated crystal structure of the samples has been determined in accordance with a four-dimensional formalism from powder neutron diffraction data. With increasing x, the modulation amplitudes in Co-Co conduction paths become less marked. The non-doped sample (x=0) exhibits the resistivity ρ=150μΩm and the thermoelectric power S=130, μV/K at 300 K. Both the ρ and S values decrease with x but the decrease in ρ values is steeper than that of the S values. The power factor S²/ρ increases with x, from 110μW/m/K² (x=0) to 140μW/m/K² (x=0.20) at 300 K. This improvement in thermoelectric performance can be explained in terms of the reduction of displacive modulation of the Co-Co conduction paths in the CoO₂ sheets.