The composition of essential oils in the seeds of Brassica juncea (L.) Coss., grown in Hebei, Shaanxi and Shandong Province was determined by gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS). We have identified twenty-two compounds constituting 89.6% by weight of the mustard oil from Hebei Province. The main components were allyl isothiocyanate (54.8%), diallyl trisulfide (9.4%), diallyl sulfide (5.5%) and 3-butenyl isothiocyanate (4.8%). Fourteen compounds representing 94.5 % by weight of the mustard oil from Shaanxi Province were identified. They were allyl isothiocyanate (68.8%), diallyl trisulfide (7.8%) and 3-butenyl isothiocyanate (4.9%). Fifteen components were identified accounting for 94.3% by weight of the mustard oil from Shandong Province. Major constituents were allyl isothiocyanate (61.3%), diallyl trisulfide (9.7%) and 3-butenyl isothiocyanate (5.9%).