Abstract: The Thymus mongolicus of Wo Yun Mountain in Yulin was experimental materials in the study. In this paper, the extraction method of essential oil from T.mongolicus with the distillation of the ultrasonic assisted was studied.Based on single factor test, the factors which affect the extraction yield of the essential oil of T.mongolicus such as extraction time, solid/solvent ratio, extraction temperature and ultrasonic power were studied by the orthogonal experimenta1. The results showed that the order of the influence factor was ultrasonic power > extraction temperature > extraction time > solid/solvent ratio. The optimal condition was extraction time 40 minutes, solvent/solid 20∶ 1 ( m L∶ g) , extraction temperature 45 ℃, ultrasonic power 200 W.Under these conditions, the essential oil of T.mongolicus extraction rate was 0.901% ± 0.1% in verification tests. This method can be used for the actual production of pepper oil to provide some theoretical reference.