In order to synthesize the partially stabilized $alpha$-Sialon, A1N and Y2O3 were added to synthesized $alpha$-Si3N4. The phase composition, mechanical properties, micro structure, etc, of the synthesized $alpha$-Sialon were investigated. Partially stabilized $alpha$-Sialon ceramics could be synthesized from the composition which was a little deviated from x=0.4, x=0.6 composition along the Si3N4.0.1Y2O3:0.9AlN tie line at 1750-180$0^{circ}C$ for 2 hrs in N2 atmosphere. It is assumed that A1N is more closely related than Y2O3 to the formation of $alpha$-Sialon, and that A1N is more easily dissolved into $alpha$-structure than into $eta$-structure. In Ya2O3-rich phase mechanical properties were observed to be poor because of formation of mellilite, grain growth, and thermal decomposition of $alpha$-Sialon. The maximum values of M.O.R, KIC and hardness are 723 MPa, 4.5MN/㎥/2 and 19.3 GPa, respectively, and they were observed for the $alpha$-Sialon ceramics sintered at 178$0^{circ}C$.