[en] Thermoelectric properties of Al-substituted Ge clathrate compounds Ba₈Al_xGe_46-x were investigated experimentally and theoretically. Polycrystalline Ba₈Al_xGe_46-x (x=12-18) was synthesized by using an arc melting and a spark plasma sintering technique. The obtained samples show the n-type behavior and the carrier concentration decreases with increasing amount of Al composition from x=12 to 16. The reasonably large power factors was obtained: 900-1000 μW/cm K² around 900 K. Theoretically electronic structure of Ba₈Al₁₆Ge₃₀ was calculated by means of a first-principles method and then thermoelectric properties were analyzed on the basis of the result of the calculated electronic structure. The calculated thermoelectric properties agree with the experimental results very well in the wide range of temperature between room temperature and 900 K