The high thermal conducting mechanism of AlN ceramics has been investigated by the relationship between microstructure and thermal conductivity of the samples which were obtained by pressureless-sintering at 1900°C for 1-24h with Y₂O₃ 2 at% addition as a sintering aid. The thermal conductivity increased gradually with increasing sintering time from 199W/m·K at 1h to 266W/m·K at 24h. Microstructure was investigated by SEM, TEM and powder X-ray diffraction together with quantitative analysis of Y and O contents. AlN grain size increased from 4μm at 1h to 12μm at 24h and the grain boundary phase gradually concentrated to triple points and the amount of this phase decreased with sintering time. Furthermore, grain boundary phase composition changed from Al₂Y₄O₉(Al₂O₃⋅2Y₂O₃) at 1h to Y₂O₃ at 24h. This change seems to occur through selective reduction and nitrization of Al₂O₃ in Al₂Y₄O₉ with an increase in sintering time. These results show that the decrease in phonon scattering at grain boundaries improves the thermal conductivity.