We have grown single-crystalline samples of Zn-substituted type-VIII clathrate Ba8Ga16Sn30 with n-type carriers by Sn-flux method. The actual compositions of the single crystals were found to be described as Ba8Ga15.8-2yZnySn30.2+y (y = 0 ∼ 0.54), where the charge balance is well maintained. As y goes from 0 to 0.42, the resistivity at 300 K decreases from 5.3 to 3.0 mΩ cm gradually, but the effective mass is essentially constant at 1.2 ∼ 1.5m0, indicating intact band structure near the conduction band minima upon Zn substitution for Ga. At elevated temperatures, the ambipolar effect on the thermal conductivity becomes less pronounced upon Zn doping, and the dimensionless figure of merit ZT for y = 0.07 and 0.42 remains at rather high values compared with the nondoped sample.