A composition containing a-$Si_3N_4$ with 5w/0 $Y_2O_3$ and 4w/0 $Al_2O_3$ was hot-pressed at 1, $650^{circ}C$ and 350kg/$cm^2$ for 1.5hrs and specimens of the same composition were pressureless-sintered at 1, 75$0^{circ}C$ for 1.5 and 5hrs. By X-ray diffraction it was found that hot-pressed specimens were consisted of $alpha$-and $eta$-$Si_3N_4$ and sintered specimens were consisted of $eta$-$Si_3N_4$ and $Si_3N-4Y_2O_3$ which was crystallized out from the grainboundary phase. The 5-hr sintered specimens had higher degree of crystallization than the 1.5 hr sintered specimens. Among these three different specimens the 5-hr sintered specimens showed the highest strength by hot MOR test at 1, 00$0^{circ}C$. The SPT diagram for the 5-hr sintered $Si_3N_4$ was constructed by measurements of the stress rate dependence of fracture strength.