We calculated the electronic structures and the thermoelectric properties for type-I and type-VIII Ba8Ga16Sn30 (BGS) clathrates. The band structures show that type-I and type-VIII BGS are indirect semiconductors with band gaps of 0.51 eV and 0.32 eV, respectively. The calculated Seebeck coefficient of n-type type-I BGS is higher than that of n-type type-VIII BGS because of the larger density of states in type-I at the bottom of the conduction band. This is in good agreement with the experimental results. We also calculated the electrical conductivity and thermal conductivity due to charge carriers. Estimated thermoelectric figure of merit, ZT, exceeds 1.0 for both types.