Three new compounds were isolated from the methanol extracts of buckwheat (Fagopyrum esculentum) hull, and their structures were determined to be 6,7-dihydroxy-3,7-dimethyl-octa-2(Z),4(E)-dienoic acid (1), 6,7-dihydroxy-3,7-dimethyl-octa-2(E),4(E)-dienoic acid (2), and 4,7-dihydroxy-3,7-dimethyl-octa-2(E),5(E)-dienoic acid (3) by NMR and MS spectroscopic analyses. These compounds at $500{mu}g$ concentration showed antimicrobial activity against Staphylococcus aureus, demonstrated by the paper disc method.