The Cu–Ga–Te ternary compounds: Cu3Ga5Te9, Cu2Ga4Te7, CuGa3Te5, CuGa5Te8, and CuGaTe2 have zinc-blende or chalcopyrite structure. The compounds except for CuGaTe2 contain vacancies in the cation site due to the valence mismatch between cation and anion, and the vacancy concentration is different between these compounds. Here we investigated the effect of the amount of vacancies on the thermoelectric (TE) properties of the Cu–Ga–Te ternary compounds. At room temperature, the presence of vacancies reduced the Hall mobility (μH) and the lattice thermal conductivity (κlat), showing that the vacancies scattered both carriers and phonons. It was found that the decreasing rate of the μH was larger than that of the κlat. Therefore, the presence of vacancies degraded the TE performance of the Cu–Ga–Te ternary compounds. In other words, CuGaTe2 without vacancies would show the best TE figure of merit. The room temperature ZT values for Cu3Ga5Te9, Cu2Ga4Te7, CuGa3Te5, CuGa5Te8, and CuGaTe2 were 2.0 × 10−2, 2.4 × 10−2, 8.9 × 10−3, 1.3 × 10−2, and 5.6 × 10−3, respectively.