In this study, the phase diagram of the AlN-Al2O3-CaO system at 1600 °C was experimentally investigated using the classical equilibration and quenching technique. The liquidus of CaO and AlN in the AlN-Al2O3-CaO system at 1600 °C was accurately determined for the first time using the mixture of pre-melted CaO-Al2O3 glass, CaO and AlN powder as the starting materials under the N2-5 vol% H2 atmosphere. The accuracy of FactSage FTOXCN database was confirmed by the comparison of the calculated diagram with the present experimental results. Thermodynamic analysis of the AlN sintering process with CaO additive was carried out using the FTOXCN database to understand the influence of CaO additive to densification of AlN.