The effectiveness of substitution for La by Ba or Ca on the physical properties of rare earth chalcogenide La₃S₄ was examined. Cubic γ -phase (Th₃P₄-type) was obtained in the range of 0≤x≤1 in La_3-xA_xS₄ (A=Ba, Ca). The lattice parameter in La_3-xBa_xS₄ system increased, while that in La_3-xCa_xS₄ system decreased with an increase of x, which is ascribed to the difference of the radius among the ions. The thermoelectric power (α) and the electrical resistivity (ρ) of La_3-xA_xS₄, which are functions of the electron concentration, increased with an increase of the temperature and the alkaline earth metal content x. A maximum value of the power factor (p=α²/ρ) at high temperature above 750K was obtained at x=0, i.e., La₃S₄. On the other hand, the thermal conductivity (κ) decreased with an increases of x. In result, a maximum value of the figure of merit (Z=α²/(ρκ)) was obtained at large value of x (x≈0.6 in La_3-x, Ba_xS₄ and x≈0.8 in La_3-xCa_xS₄).