Phenethyl isothiocyanate (PEITC) was obtained from horseradish. The preparation procedure was as follows: the horseradish powder was hydrolysed in the water first, and then, after filtration, the residue was extracted by petroleum ether; finally, PEITC was isolated by silica gel column. The structure of PEITC was identified by IR, MS, ¹H-NMR and ¹³C-NMR chromatography methods. The inhibitory activities of PEITC against Gibberella zeae, Xanthomonas axonopodis pv . citri, Cytospora sp . and Phytophthora capsisi showed that PEITC had good inhibition effects. The EC₅₀ values of G. zeae, X. axonopodis pv . citri, Cytospora sp . and P. capsisi were 13.92, 1.20, 0.73 and 3.69 µg mL⁻¹, respectively.