The extracts from twenty-eight species of Thai medicinal plants were screened for anti-lipase activity at a final concentration of 5 µg/ml. The stems of Neagia wallichiana showed highly potent activity against pancreatic lipase (86.60±0.66%). After the isolation to determine the active compounds from this plant extract, seven pure compounds were isolated including of β-sitosterol, nagilactone A, inumakiol D, totarol, ferruginol, bis-2-ethylhexyl-phathalate, and nagilactone E-3-O-β-glucopyranoside. These chemical structures were elucidated based on extensive study of various spectroscopic data including UV, IR, MS and NMR, and comparison with previously reported data. The finding of this study found that the three active compounds were totarol, ferruginol, and bis-2-ethylhexyl-phthalate which were potent inhibitors with the IC50 values of 31.65±0.19, 30.98±2.24, and 32.75±1.93 µM respectively. However, these compounds were less potent than orlistat and gallocatechin gallate which their IC50 values were 15.86±4.79 and 198.27±1.47 nM respectively. Totarol and ferruginol had the same core skeleton of the structure as abietane-type diterpenoids which had previously been reported for anti-lipase activity. However, this was the first report for anti-lipase activities of these three active compounds. Furthermore, this was the first report on a novel compound as nagilactone E-O-β-glucopyranoside.