Abstract: Volatile oil from the shoot of Wedelia trilobata L., an invasive alien weed in South China, was extracted by steam distillation and its fungitoxicity to Magnaporthe grisea Barr., Rhizoctonia solani (Oryza sativa L.) and Rhizoctonia solani (Zea mays L.) investigated. The chemical constituents of the volatiles were identified by GC-MS analysis. The volatiles inhibit mycelial growth of the above 3 pathogens at the maximum of 54.8%, 32.9% and 56.3% respectively. 34 constituents are found in the volatile oil, of which 29 identified by mass spectrum constitute 90.88% volatile oil. The main constituents of the volatile oil are 2,6,6-Trimethyl-Bicyclo3.1.1-2-heptene (25.16%), 2,7,7-Trimethyl-Bicyclo2.2.1-2-heptene (22.90%) and γ-terpinene (8.98%). The results indicate that volatiles of W. trilobata have strong antibiotic effect on native pathogens. This may be an important mechanism for the successful invasion of the exotic plant.