Diabetes mellitus is a disease due to chronic heterogeneous metabolic disorders with complex pathogenesis. The global prevalence of diabetes patients aged 20-79 years in 2021 is estimated to be 10.5% (536.6 million people) and will increase to 12.2% (783.2 million people) in 2045. In order to reduce the prevalence of diabetes, it is important to reduce the entry of glucose into the blood circulation. Mangosteen peel has xanthone compounds that act as α-amylase inhibitors in saliva. If the α-amylase enzyme is inhibited, polysaccharides in food will not be broken down into Glucose and it will reduce the absorption of glucose into the blood vessels. This study aims to make oral thin film preparation that works locally in saliva, to evaluate the quality of the preparation and the activity of α-amylase enzyme inhibitor using UV-Vis spectrophotometer. This was experimental research by making mangosteen peel extract using maceration method using 96% ethanol solvent. Oral thin film formulation with varying concentrations of mangosteen peel extract, namely 0 mg (F0), 50 mg (F1), 100 mg (F2) and 150 mg (F3). The data obtained were analyzed using one way-ANOVA and Post Hoc LSD test with SPSS 16.0 and 95% significance level. The results showed that F3 was the best formulation that met BPOM standardized for oral thin film preparation. The variety in concentration of mangosteen peel extract affects the results of the organoleptic test, preparation weight test, pH value test and disintegration time test (p <0.05), but has no effect on the value of the film thickness test results (p>0.05). Oral thin film preparation of mangosteen peel extract has α-amylase inhibitor activity (p<0.05) with the highest inhibitor activity in formulation 3 (150 mg mangosteen peel extract).