Type-VIII clathrate Ba8Ga16Sn30 is a promising thermoelectric material in the 400–600 K range. We report on the effects of Ge substitution for Sn on both crystal growth and thermoelectric properties. In a series of samples prepared from the initial composition of Ba8 Ga40 Sn30−X GeX (0 ≤ X ≤ 6), the size of single crystals reaches the maximum of 8 mm at X = 0.5. The atomic composition of single crystals is found to be described as Ba8Ga15.9Sn30.1−xGex (0 ≤ x ≤ 4.73). Ge substitution results in the increase in the positive Seebeck coefficient and electrical resistivity by 1.3 and 2 times at the maximum, respectively, whereas the hole carrier density does not change substantially. Taken together with the thermal conductivity data, the maximum of the dimensionless figure of merit ZTreaches 0.87 at 540 K for x = 0.07.