[en] Yttrium is one of the rare earth metals which is used to improve properties of metals. This study was conducted to determine the effect of Yttrium (Y) on high-temperature oxidation resistance of zirkonium alloy 4 by using isothermal oxidation method at temperatures of 900 °C and 1000 °C. This study employs 3 varied yttrium concentration of 0 wt %, 0.5 wt% and 1 wt% as dopping material for zircaloy-4 (Zr, Sn, Fe, and Cr). The isothermal oxidation process was conducted at temperatures of 900 °C and 1000 °C for 9 hours heating of samples as cast in a furnace. Tests were done to the samples after oxidation process, which include metallographic testing, hardness testing, X-Ray Diffraction (XRD) testing and oxide thickness measurements. Test results show that the varying content of yttrium does not significantly affect the value of hardness, but there is a tendency that greater yttrium content decreases hardness value. The addition of yttrium affects the oxidation resistance at high temperatures, which can be seen from the graph of oxide thickness as well as the morphology of the oxide surface of each sample. (author)