The composition of essential oils in the leaves of three cultivars (Ban-Chung-Gat, Chung-Gat and Dolsan-Gat) of Brassica juncea L. were analyzed and their antifungal activity were investigated in this study. Allyl isothiocyanate, 2-phenyl ethyl isothiocyanate, 4-isothiocyanato-1-butene, 5-methyl isothiazole, benzene acetaldehyde, benzene propane nitrile and beta-ionone have been identified in all of the experimented oils. The main component of the oils from Ban-Chung-Gat and Chung-Gat was 2-phenyl ethyl isothiocyanate while allyl isothiocyanate was the representing compound in the oil of Dolsan-Gat. The antifungal activities of the oils were tested by micro broth dilution method and disc diffusion method. As the result the oils exhibited significant inhibiting activities against Aspergillus niger, A. flavus, Trichoderma viride, Candida albicans, C. utilis, C. tropicalis, Cryptococcus neoformans, Trichosporon mucoides, Trichophyton tonsurans and Geotrichum capitatum.