Garcinia mangostana rind containing alpha-mangostin has the potential to be used as an antioxidant. This study aimed to determine the physicochemical properties and stability of alpha-mangosteen in the 50% ethanol extract of mangosteen rind as a parameter in the pre-formulation study. Physicochemical properties were evaluated for specific and nonspecific extract parameters, determining the optimum wavelength and solubility test. The stability test was carried out at a temperature of 45±5°C with relative humidity 75±5% for 21 days, in acidic and alkaline conditions. The alpha-mangostin content was analyzed using a UV-Vis spectrophotometer. The results showed that the extract was purplish brown, has an aromatic smell, and a bitter taste. The dissolved compound in water and ethanol were 62.54±1.09% and 87.05±0,43%, respectively, the specific density of 5% extract was 1.036, loss on drying 6.66±0,11%, ash content 5.07±0.23%, ash content insoluble in acidic was 0.13±0.02%, alpha-mangosteen content in the extract was 3.85±0.03%. The solubility test showed that alpha-mangosteen in the extract has a solubility value of 1: 16064 in water. The stability test showed that the levels of alphamangostin decreased significantly (p≤ 0.05) at 45±5°C and relative humidity 75±5% for 21 days. Mangosteen rind extract was unstable in the extremely acidic and alkaline conditions.