The hydrolytic conditions of thioglucosides (TGD), which are the precursors of the essential oil ofBrassica juncea(L.) Coss., and the effects of time, temperature, pH and ascorbic acid added on the hydrolysis of TGD were determinated. The optimum hydrolytic conditions for the essential oil production from TGD inB. junceaare time, 120 min; temperature, 70°C; pH, 4.5; and ascorbic acid, 0.02 mg/g. The oil was analyzed by GG/MS. Fourteen components were identified, representing 85.9% of the oil. The main components found in the oil were allyl isothiocyanate (43.8%), 3-methylthiopropyl isothiocyanate (9.4%), 3-butenyl isothiocyanate (4.8%), butyl isothiocyanate (4.7%), phenyl isothiocyanate (3.7%) and 2-phenylethyl isothiocyanate (3.7%).