$eta$-Sialon powder was synthesized by the simultaneous reduction and nitridation of the mixed powder of Hadong kaolin and Kimcheon quartzite, using graphite as a reducing agent. The synthesized $eta$-Sialon powder (Z=1) was hot-pressed at 175$0^{circ}C$, for 90min under 30MPa in N2 atmosphere, after Yttria and YAG composition material were added as sintering agents. The effects of grain-boundary crystallization on high-temperature mechanical properties of $eta$-Sialon ceramics were investigated. Strength degradation was observed at above 1,00$0^{circ}C$ for the $eta$-Sialon (Z=1)-8wt% Y2O3 composition, but it was not observed up to 1,20$0^{circ}C$ for the $eta$-Sialon-8wt% YAG composition which was annealed at 1,40$0^{circ}C$ for 4 hours in N2 atmosphere. After the $eta$-Sialon-8wt% YAG composition was annealed, the decrease of fracture toughness was observed.