The powders of the system Si3N4-Y2O3-AlN were prepared using Si(OC2H5)4 and YCl3.6H2O together with commercial AlN powder. $alpha$-Si3N4 was prepared by the carbothermal reduction and nitridation of the hydrolyzed gel at 135$0^{circ}C$ for 10h in N2 atmosphere. YCl3.6H2O was observed to be changed to Y2O3 during the reaction. $alpha$-Sialon(X=0.2, 0.4, 0.6) ceramics were obtained by hot-pressing the Si3N4-Y2O3-AlN mixture at 178$0^{circ}C$ for 1h under 30 MPa. The content of $alpha$-Sialon increased with increasing metal solubility(x value) and $alpha$-Sialon single phase was obtained at the metal solubility of 0.6. With increasing metal solubility, flexural strength, fracture toughness and thermal shock resistence were decreased, while the microhardness was increased. Large elongated $eta$-Si3N4 grains were mainly observed at lower metal solubility. Mechanical prorerties of the sintered ceramics with X=0.2 were measured as follows : flexural strength ; 650 MPa, fracture toughness ; 3.63 MN/m3/2, hardness ; 14.7 GPa, thermal shock resistence temperature ; 58$0^{circ}C$.