The objective of the work in hand was appraising the phenolic composition and the in vitro antioxidant activity by reducing power and lipid peroxidation inhibition essays as well as the anti-proliferative activity on C6 and Hela cancer cell lines of butanol extract of the aerial parts of Euphorbia helioscopia collected in humid areas in Algeria. The results revealed that butanol extract showed a good antioxidant activity in all the tested methods. The total phenolics and flavonoids were relatively high including 156.21±4.55 mg GAE (gallic acid equivalent)/g and 22.08±0.81 mg QE (quercetin equivalent)/g, respectively. Phenolics were analyzed by HPLC-TOF-MS, and identified by comparing their retention times, UV-Vis absorption spectra and mass spectra with authentic standards. It was shown that gentisic acid was the major component of the extract. The extract seemed to possess important antioxidant potencies and phenol contents. The ferric reducing activity was 1.396 at 1000 µg/mL, and the lipid peroxidation inhibition was 44.38 % at 500 µg/mL. The extract exerted a moderate inhibition of proliferation against Hela cell, and no inhibitory effect against C6.