The composition of the volatile oil from Sinapis Semen, dried seeds of Brassica hirta (Cruciferae), have been investigated using capillary GC and GC/MS. The important odor-active compounds in the oil were also detected by means of GC-olfactometry (GC-O) and aroma extract dilution analysis (AEDA). The oil was found to contain 68 components, representing 95.36% of the total oil. The main constituents were 4-hydroxy-benzeneacetonitrile (29.63%), palmitic acid (14.55%), linoleic acid and oleic acid (9.67%). The AEDA and GC-O analyses showed that heptanal, phenylethylalcohol, (2E)-octenal and (2E, 4E)-dodecadienal are to a large extent responsible for the characteristic flavor of Sinapis Semen.