Electrical and thermoelectric properties of the undoped (100-x)% Bi₂Te₃-x% Sb₂Te₃ (x≥66.7) pseudo-binary single crystals grown by the zone melting method were measured over the temperature range from 77 to 600K. For the (100-x)% Bi₂Te₃-x% Sb₂Te₃ (x≥66.7) alloys, the temperature dependence of the Hall mobility was T^(-1.0) regardless the Sb₂Te₃ contents. The temperature dependences of (m^*/m_o)^(3/2)·μ_c and the effective mass were proportional to T^(-1.5) and T^(-1/3), respectively. The saturated hole concentration decreased with increasing temperature and the slope of the Seebeck coefficient changed at about 350K, which implied the two sub-bands in the valence band. While the 33.3% Bi₂Te₃-66.7% Sb₂Te₃ alloy exhibited the highest figure-of-merit among the undoped Bi₂Te₃-Sb₂Te₃ alloys, the maximum figure-of-merit could be expected fur the 22.5% Bi₂Te₃-77.5% Sb₂Te₃ alloy with optimizing the carrier concentration.