The four new and four known sesquiterpenoid derivatives 1 – 4 and 5 – 8, respectively, were isolated from the air-dried roots of Ferula mongolica. The structures of these compounds were determined by spectroscopic methods and found to be rel-(2R,3R)-2-[(3E)-4,8-dimethylnona-3,7-dienyl]-3,4-dihydro-3,8-dihydroxy-2-methyl-2H,5H-pyrano[2,3-b][1]benzopyran-5-one (1), rel-(2R,3R)-2-[(3E)-4,8-dimethylnona-3,7-dienyl]-2,3-dihydro-7-hydroxy-2,3-dimethyl-4H-furo[2,3-b][1]benzopyran-4-one (2), rel-(2R,3R)-2-[(3E)-4,8-dimethylnona-3,7-dienyl]-2,3-dihydro-7-hydroxy-2,3-dimethyl-4H-furo[3,2-c][1]benzopyran-4-one (3), rel-(2R,3R)-2-[(3E)-4,8-dimethylnona-3,7-dienyl]-2,3-dihydro-7-methoxy-2,3-dimethyl-4H-furo[3,2-c][1]benzopyran-4-one (4), (4E,8E)-1-(2-hydroxy-4-methoxyphenyl)-5,9,13-trimethyltetradeca-4,8,12-trien-1-one (5), the rel-(2R,3S) diastereoisomer 6 of 2, the rel-(2R,3S) diastereoisomer 7 of 4, and (4E,8E)-1-(2,4-dihydroxyphenyl)-5,9,13-trimethyltetradeca-4,8,12-trien-1-one (8). These compounds were tested as inhibitors against the enzyme α-glucosidase. The compounds 1 – 6 and 8 exhibited significant inhibitory activity and, therefore, represent a new class of α-glucosidase inhibitors.