1 Sustainable Development Proceedings, Volume 1, 2023 This in open article under the Creative Commons Attribution 4.0 International (CC BY 4.0) license. Peer-review under responsibility of the organizing committee 1st International Conference on ASEAN Sustainable Development (ICASD 2023). Chemical constituents from Citrus grandis Linn. and their biological activities Uraiwan Phetkul 1 *, Auyporn Vongkul 1 , Kewalee Chaichan 1 , Supakit Paosen 2,3 , and Supayang Piyawan Voravuthikunchai 2,3 1 Faculty of Science and Technology, Rajamangala University of Technology Srivijaya, Thungsong, Nakhon Si Thammarat 80110, Thailand. 2 Center of Antimicrobial Biomaterial Innovation-Southeast Asia, Faculty of Science, Prince of Songkla University, Hat Yai 90110, Thailand. 3 Natural Product Research Center of Excellence, Faculty of Science, Prince of Songkla, University, Hat Yai, Songkhla 90112, Thailand. *Correspondence: uraiwan.p@rmutsv.ac.th Abstract This research aimed to study the chemical constituents and their biological activity of the peels of Citrus grandis Linn. (Tubtim Siam Pomelo). The peels of C. grandis Linn. were immersed in acetone at room temperature. After evaporation, the acetone extract was purified with column chromatography (CC) to give a flavonoid ( 1). The structure of pure compound (1) was elucidated by analyses of spectroscopic data and comparing their spectroscopic data with the previously published data. Crude acetone and 1 have been evaluated for antioxidant, anti-tyrosinase, and antimicrobial. Compound 1 has DPPH radicals and tyrosinase with IC50 0.123 ± 0.004 and 0.357 ± 0.117 mg/mL, respectively. Keywords: Citrus grandis Linn, flavonoids, antioxidant activity, anti-tyrosinase activity, antimicrobial activity