In this study, p-type Ba₈Cu₆Ge₄₀ and n-type Ba₈Cu₅Ga_xGe_41−x−yP_y (x=0, 1, 2, 3; y=0, 1) were synthesized by arc melting and spark plasma sintering processes, and their thermoelectric properties were investigated. p-type Ba₈Cu₆Ge₄₀ shows a relatively high Seebeck coefficient due to the many-valley effect in the valence band, and its thermoelectric figure of merit ZT is about 0.78 (about 720 K), which is higher than that of n-type Ba₈Cu₅Ga_xGe_41−x−yP_y. On the other hand, n-type Ba₈Cu₅Ga_xGe_41−x−yP_y has a lower electron concentration than the comparative material Ba₈Cu₅Ge₄₁ due to simultaneous Ga−P substitution, and ZT at x=2 is about 0.60 (about 670 K), which is higher than the previously reported value.