Aluminum nitride (AlN) ceramics were fabricated by pressureless sintering with various sintering additives such as 5 mass % Y₂O₃, 2 mass % Y₂O₃–3 mass % Al₂O₃, 3 mass % Y₂O₃–1 mass % CaO–0.08 mass % B, and 6 mass % Y₂O₃–2 mass % CaO–0.16 mass % B. The relative density, phases present, lattice parameter c, and thermal conductivity of the samples were evaluated. The oxygen concentration in AlN lattice was estimated from the lattice parameter c by using the relation between the oxygen concentration and the lattice parameter c of AlN. To analyze the effects on the thermal conductivity of the samples, we plotted the oxygen concentration in AlN lattice estimated from the lattice parameters c versus the thermal resistivity, which is inverse of the thermal conductivity measured in this work. The data of the samples sintered with 5 mass % Y₂O₃ and 3 mass % Y₂O₃–1 mass % CaO–0.08 mass % B located on the plot of the relation between oxygen concentration in AlN lattice and thermal resistivity of AlN presented by Slack (Slack’s line) or Harris et al. (Harris’s line), respectively. In contrast, the data of the samples sintered with 2 mass % Y₂O₃–3 mass % Al₂O₃ and 6 mass % Y₂O₃–2 mass % CaO–0.16 mass % B, which contain excess grain boundary phases, located above the Harris’s line. It was suggested that the thermal conductivity of the samples could be influenced by the density, the oxygen content in AlN lattice, and the presence of grain boundary phases.