Millimeter-wave sintering of aluminum nitride for various contents of Yb₂O₃ sintering aid was performed under nitrogen atmosphere. Densification temperature to obtain more than 95%T.D. nonlinearly depended on the Yb₂O₃ content and it was found that the lowest temperature for a densification over 96%T.D. was 1600°C in the 5 wt% Yb₂O₃-added A1N. Dependence of densification temperature on the Yb₂O₃ content was closely related with liquidus temperature in the binary Yb₂O₃-Al₂O₃ phase diagram. The low densification temperature was attributed to the eutectic temperature between Yb₂O₃ and Yb₃Al₅O₁₂. Thermal conductivity of sintered A1N increased drastically with the content of Yb₂O₃ up to 3 wt%. No remarkable improvement in the thermal conductivity in the range over 3 wt% Yb₂O₃ was caused by excess amount of Yb₂O₃ in the sintered AIN.