After sintering Si3N4 containing 20wt% of variable composition ratio of Y2O3 and Al2O3 at 1$600^{circ}C$, the specimens were annealed at 125$0^{circ}C$ and 135$0^{circ}C$ for 5, 10, 15 hours in order to crystallize the remanining oxynitride glass phases. The main grain-boundary crystalline phases in the Si3N4-Y2O3-Al2O3 system were melilite and YAG. By annealing 15hrs. at 125$0^{circ}C$, almost all of the glasses were crystallized. During the growth of melilite, lattice volyume of $eta$-Si3N4 was increased as Al3+ and O2- ions in the oxynitride glass diffuse into $eta$-Si3N4 lattice, but during the growth of YAG, lattice volume of $eta$-Si3N4 was decreased by reverse diffusion of Al3+ and O2- ions. In case of crystallization of glass phase to melilite, thermal expansion of sample was decreased, but in case of crystallization to YAG, inverse phenomen on was observed.