The specimens which were prepared from $alpha$-Si3N4 with additions of YAG(3Y2O3.5Al2O3)-10 wt% and various AlN contents were sintered in N2 atmosphere at 1$700^{circ}C$ The effect of $alpha$,$eta$-solid solution contents and sintering time on mechanical properties were investigated. As the content of $eta$-solid solution and sintering time increased the hardness is reduced but the hardness of specimen sintered over 10 hours is constant irrespective of sintering time. While the fracture toughness increased with increasing of $eta$-solid solution and sintering time. The fracture toughness of specimen with 80% $eta$-solid solution content increased from 3.89 to 6.66 MPam1/2 with sintering sintering up to 20 hours/ But the amount of increased fracture toughness of specimen with below 20% $eta$-solid solution content is not significant.