We investigate the effects of Yb2O3 and calcium aluminosilicate (CAS) glass as sintering additives on the sintering behavior of AlN. The AlN specimens are sintered at temperatures between 1700oC and 1900oC for 2 h in a nitrogen atmosphere. When the Yb2O3 content is low (within 3 wt.%), an isolated shape of secondary phase is observed at the AlN grain boundary. In contrast, when 3 wt.% Yb2O3 and 1 wt.% CAS glass are added, a continuous secondary phase is formed at the AlN grain boundary. The thermal conductivity decreases when the CAS glass is added, but the sintering density does not decrease. In particular, when 10 wt.% Yb2O3 and 1 wt.% CAS glass are added to AlN, the flexural strength is the highest, at 463 MPa. These results are considered to be influenced by changes in the microstructure of the secondary phase of AlN.