Fennel, commonly used in traditional Chinese medicine, is known as Foeniculum vulgare Mill. And its clinical application has been shown to target many biological systems including gastroenterology, endocrinology, gynecology and respiratory. The main constituent of the fennel essential oil is trans-anethole, which has been described to have anti-inflammatory and antibacterial activities. The aim of the present study was to define the anti-inflammatory influence in acute lung injury (ALI)-bearing mice. For 3 days, ALI-bearing mice were induced by lipopolysaccharide (LPS) suspension in normal saline (24 mg/kg). On the fourth day, the trans-anethole was administrated (36.4, 72.8 or 145.6 mg/kg) as well as dexamethasone (5 mg/kg) once per day for 7 consecutive days in mice. Following the completion of drug administration mice were sacrificed. Hematoxylin and eosin staining was performed in the lung paraffin section, for comparisons between monocyte and eosinophil cells in bronchoalveolar lavage fluid. The relative gene expression of interleukin (IL)-10 and IL-17 was determined by reverse transcription-quantitative polymerase chain reaction. These two cytokines and the proportion of T helper 17 (Th17) cells and T regulatory (Treg) cells were determined by flow cytometry. The main constituent of fennel, trans-anethole, eliminated LPS-induced histopathological changes, decreased the number of inflammatory cells and resulted in a notable reduction in IL-17 mRNA expression. In addition, trans-anethole increased IL-10 mRNA expression in isolated lung tissues and resulted in a marked elevation in Treg cells and reduction in Th17 cells in spleen tissues. The results of the present study indicated that the main constituent of fennel, trans-anethole may be an anti-inflammation component, which influenced the regulation of Th17/Treg responses. Therefore, this medicinal herb may support a healing effect on diseases of inflammatory.