In order to evaluate the oxidation behavior of $eta$-Sialon, $eta$-Sialon ceramics was prepared from Si3N4, Al2O3, AlN and Y2O3 system. The specimens were oxidized in an oxygen atmosphere at 1, 20$0^{circ}C$ for 9days. Oxidation behavior was evaluated by weight gain oxidation process, surface roughness. Microscopy, EDX and X-ray diffraction analysis were also used for the evaluation. The weight and surface roughness ofoxidized specimens were increased with increasing the oxidation time. Oxidized products were mullite, $alpha$-cristobalite, yttrium aluminum oxide and yttrium silicate oxide.