In the past decade, nanofluids as working fluids have been prevalent due to better heat transfer efficiency than conventional working fluids. However, the problem with using nanofluids is the precipitation and hazardousness of nanoparticles. Therefore, plant extracts-based synthesis procedures have drawn consideration over traditional methods like physical and chemical to synthesize nanofluid. This research synthesized silver nanoparticles (AgNPs) suspended in fluid at various conditions by extracts from mangosteen peels as reducing and capping agents. Nanoparticle formation was confirmed by observing the color change of the mixture of silver nitrate from yellow to a brown colloidal suspension after adding the plant extract. In addition, UV-Vis spectroscopy, SEM, and EDX were used to analyze the synthesized nanoparticles. The prepared nanofluid shows a characteristic absorption peak band between 440 and 460 nm wavelengths. The particle size was obtained by SEM, which is spherical with an average diameter of less than 10 nm and is very well dispersed in the fluid. Furthermore, the EDX mapping images confirmed that the silver nanoparticles were distributed throughout the liquid.