Magnolol, a tradition Chinese herb, displays an array of activities including antifungal, antibacterial, and antioxidant effects. To investigate the protective effect of magnolol on lipopolysaccharide (LPS)-induced acute lung injury (ALI) in mice. ALI was induced in mice by intratracheal instillation of LPS (1 mg/kg). The mice received intratracheal instillation of magnolol (5 μg/kg) 30 min before LPS administration. Pulmonary histological changes were evaluated by hematoxylin-eosin stain and lung wet/dry weight ratios were observed. Concentrations of tumor necrosis factor (TNF)-α and interleukin (IL)-1β, and myeloperoxidase (MPO) activity were measured by enzyme-linked immunosorbent assay. Expression of cyclooxygenase (COX)-2 in lung tissues was determined by Western blot analysis. Magnolol pretreatment significantly attenuated the severity of lung injury and inhibited the production of TNF-α and IL-1β in mice with ALI. After LPS administration, the lung wet/dry weight ratios, as an index of lung edema, and MPO activity were also markedly reduced by magnolol pretreatment. The expression of COX-2 was significantly suppressed by magnolol pretreatment. Magnolol potently protected against LPS-induced ALI and the protective effects of magnolol may attribute partly to the suppression of COX-2 expression.