In this study, we have investigated new plasma resistant materials with less usage of rare-earth oxides than $Y_2O_3$ which is currently used in the semiconductor industry. We observed the stability ranges of $(Gd{cdot}Y)_3Al_5O_{12}$ and $(La{cdot}Y)Al_{11}O_{18}$ ternary systems, and measured their etch rates under typical fluorine plasma. $(Gd{cdot}Y)_3Al_5O_{12}$ system showed an extensive solid solution up to 80 mol% gadolinium, but $(La{cdot}Y)Al_{11}O_{18}$ showed a negligible substitution between rare-earth ions, which can be explained by the differences between the ionic radii. The etch rates depended on the total amount of rare-earth oxides but not on the substitution of the rare-earth ions. When the specimen was examined using XPS after the exposure to fluorine plasma, the strong surface fluorination was observed with a shift of the binding energy to higher energy.