Watermelon is a fruit that can increase endurance and is rich in antioxidants because it contains a source of lycopene carotenoids, rich in phenolic antioxidants, cucurbitacin E, triterpene, and amino acids in unusual amounts such as L-arginine and citrulline. To facilitate its use, it is made in the form of syrup preparations that are expected to have an effect as an immunostimulant. The purpose of this study is to find out the immunostimulant effect of watermelon peel. This study was conducted by formulating watermelon peel extract in three concentration variations, namely FI (5%) FII (10% extract) and FIII (15% extract). Preparation of watermelon peel extract syrup obtained next is carried out accelerated stability test. Phagocytosis testing is carried out by calculating the activity phagocytosis of macrophage cells. Data analysis using the Graphad Prism® app. The results obtained are that the syrup formula of watermelon peel extract is stable after storage and can have an effect as an immunostimulant.