Stability of the $eta$-sitosterol glycoside from Schima wallichii sp. at various physical conditions were investigated, mutagenecity of the steroid saponin was determined by Ames test. When exposed in pH 3 to pH 8, the $eta$-sitosterol glycoside was stable on antimicrobial activity against yeasts. The antimicrobial activity of the $eta$-sitosterol glycoside also stable in high temperature, $N_2$, $O_2$ gas and light exposure, and metal ion. Ames test result revealed that $eta$-sitosterol glycoside did not have any mutagenic activity. These results suggest that the $eta$-sitosterol glycoside might be a promising candidate as a natural antimicrobial compound.