The effect of substitution for La by Ba or Ca on the thermoelectric properties of rare earth chalcogenide La₃S₄ was examined. Cubic 7 -phase (Th₃P₄-type) was obtained in the range of 0 ≤y≤1 in La_3-yBa_yS₄ and La_3-yCa_yS₄. With an increase of y, the lattice parameter in La_3-yBa_yS₄ increases, while that in La_3-yCa_yS₄ decreases. The electrical resistivity (r) and thermoelectric power (α) of La_3-yBa_yS₄ and La_3-yCa_yS₄ increase with an increase of y. The maximum power factor α²/r was obtained in La_2.8Ca_0.2S₄ at 500°C. It seems that there is the most suitable value of y for the power factor.