Red onion skin (Allium cepa L.) is a type of household waste that is rarely used by the public. Based on previous research, shallot skin contains flavonoid compounds which have the potential to act as antioxidants. This study aims to determine the concentration of shallot skin extract that is able to inhibit 50% of DPPH absorbance. This research uses an experimental type of research. The onion skin was extracted, photochemical screening was carried out, and the anti-free radical IC50 was determined. The yield of onion skin extract obtained was 5.21%. Based on this research, it is known that red onion skin extract contains polyphenols, flavonoids, saponins. The results of the antioxidant activity test for onion skin extract can be seen from the IC50 value, which is 29.9 ppm, while the IC50 for asam askorbat is 14.9 ppm. The IC50 value of the onion skin extract obtained is included in the group of very strong antioxidants.