This study was conducted to investigate the effect of ethanol acidification on the antioxidant properties of Morinda citrifolia leaf (MCL) extract and its catechin derivatives. Four different ethanol (100%, 99.5%, 70%, 50%) with or without 0.5% acetic acid were used for extraction. The antioxidant profile was studied with DPPH radical scavenging activity, FRAP and TPC. The quantification of catechins in MCL was performed using HPLC, and the identification of catechins derivatives was performed with Ultra UPLC-TWIMS-QTOF. The results showed that an extraction solvent composed of 70% ethanol: 29.5% water: 0.5 % acetic acid exhibited the highest DPPH percentage of inhibition (86.12±2.96%) and highest TPC value with 97.80±0.25 mg GAE/g extract, while 100% ethanol acidified with 0.5% acetic acid showed highest FRAP antioxidant power with 1.31±0.05mg FSE/g extract. All eight types of catechins were identified in MCL and the most total catechins were quantified in 70% ethanol: 29.5% water: 0.5 % acetic acid at 153.57mg/g. The catechin derivatives identified included epigallocatechin-3-O-gallate (EGCG), epigallocatechin (4β, 8)-gallocatechin, gallocatechin (4α→8)-epicatechin, catechin-3-O-gallate (CG) and epigallocatechin (EGC). The results suggest that acidification improves the extraction of polyphenols as well as catechin content.