From the lichen Parmotrema praesorediosum (Nyl.) Hale, Parmeliaceae, collected in Vietnam, five new (1, 2, 3, 5, 6) and one known (4) γ -lactonic acids were isolated. The structures of the new compounds were established by 1D and 2D NMR spectroscopy, as well as high resolution-electrospray ionization (HR-ESI)-MS analysis. The absolute configuration of new compounds was determined by ultraviolet circular dichroism spectroscopy. Compounds were evaluated for their cytotoxic activity against HeLa (human epithelial carcinoma), NCI-H460 (human lung cancer) and MCF-7 (human breast cancer) cell lines at the concentration of 100 μg/mL. Among six compounds, 1 possessed medium activity against MCF-7 cell line with the inhibition of 52.5%. Meanwhile, the rest showed no cytotoxic activity against three surveyed cancer cell lines. Keywords: Aliphatic acids; Lichen; Parmeliaceae; Parmotrema praesorediosum.