Effect of Ga addition on the thermoelectric properties of Ba-Ge type-III clathrate has been investigated as a function of Ga content and temperature. The substitution of Ga atom for Ge atom leads to the decrease of carrier (electron) concentration. Electrical conduction is of n-type for all clathrate compounds investigated and the values of electrical resistivity and Seebeck coefficient increase with the increase in the Ga content and in temperature. Both electronic and lattice thermal conductivity decrease with the increase in the Ga content because of the decreased carrier concentration and the increased extent of the rattling motion of Ba atoms encapsulated in open-dodecahedron, respectively. A very high ZT value of 1.25 is obtained at 670°C for Ba24Ga 15Ge85.