Using group-III atom Al as doping element, Ba8Ga16AlxGe30-x (x=1.0, 2.0, 3.0, 4.0, and 5.0) type-I clathrates with different Al content were synthesized by combining melting reaction with spark plasma sintering method. The effects of Al content on thermoelectric properties are investigated. X-ray diffraction patterns and Rietveld analysis reveal that the compounds prepared by this method are type-I clathrates and Al atom preference for the 6c site. The Al substitutions do not affect the atomic displacement parameters (ADPs) of framework atoms (Ge/Ga) and filled atoms (Ba) compared to that of Ba8Ga16Ge30, and the ADPs of Al are nearly equivalent to that of other framework Ge/Ga atoms. All specimens exhibit the behavior of the p-type conduction. The carrier concentration and electrical conductivity increase while Seebeck coefficient decreases with the increasing Al content for the specimens with ≤4.0. Ba8Ga16Al3.0Ge27.0 compound possesses the relatively lower lattice thermal conductivity κL due to the mass fluctuation between Al atoms and other atoms in the framework; it is as low as 0.96 W/mK at 300 K. The maximum ZT value of 0.61 is obtained at 760 K for Ba8Ga16Al3.0Ge27.0.