We evaluated the leaf essential oil in whole or fractions of Dysphania ambrosioides with respect to their resistance to wood decay fungal activities in vitro of 4 fungi. The main ingredients with the greater anti-wood decay capability were also identified. Fresh leaves of D. ambrosioides were hydrodistillated in a Clevenger-type apparatus and the resulting oil characterized using GC-FID and GC-MS instruments. The essential oil was found to consist of Î±-terpinene (30.5%), p-cymene (17.3%), carvacrol (16.2%), and ascaridole (15.1%). The oil showed resistance to wood decay activity of Trametes versicolor, Phanerochaete chrysosporium, Phaeolus schweinitzii, and Lenzites sulphureu. The oil had excellent resistance to wood decay fungal activities, and the active compounds were shown to be carvacrol and ascaridole.