The effect of AlN on the thermal conductivity of aluminum oxide pressurelessly sintered at nitrogen atmos-phere was investigated. Increasing aluminium nitride content up to 1~10 mol% the thermal conductivity of $Al_{2}O_{3}$-AlN system was singnificantly decreased and was constant with adding 20 and 25 mol% aluminium nitride. The thermal conuctivity of $Al_{2}O_{3}$ containing 1~10 mol% the thermal conductivity of $Al_{2}O_{3}$-AlN system was singificantly decreased and was constant with adding 20 and 25mol% aluminum nitride. The thermal conctivity of $Al_{2}O_{3}$ containing 1~10 mol% AlN showed a maximum at $1700^{circ}C$ and decrea-sed with increasing sintering tempertures. This phenomenon was attributed to $alpha$-$Al_{2}O_{3}$ and ALON formed by reacting $Al_{2}O_{3}$ with AlN up to $1700^{circ}C$ and the secondary phases such as ${gamma}$-ALON ($9Al_{2}O_{3}$.AlN)and $Phi$($5Al_{2}O_{3}$.AlN) phase above $1750^{circ}C$ The thermal conductivity of $Al_{2}O_{3}$ containing 20 and 25 mol% AlN showed maximum value at $1800^{circ}C$ Both $alpha$-$Al_{2}O_{3}$ and ALON existed up to $1600^{circ}C$ value at $1800^{circ}C$ Both $alpha$-$Al_{2}O_{3}$ and ALON existed up to $1600^{circ}C$ while only AlON phase existed above $1650^{circ}C$.