Objective： To evaluate the effect of water channel aquaporin 4 （AQP4） on bleomycininduced lung fibrosis in mice. Methods： In wild type and AQP4 gene knockout （AQP4-/-） mice， lung fibrosis was induced by injection of bleomycin （3 mg/kg） into the trachea and saline injection was used as a control. At d3， 7， 14， 28 after bleomycintreatment， mice were randomly sacrificed in batch and the lung coefficient was determined. Serum levels of TGFβ1 and TNFα were measured by ELISA and hydroxyproline contents in lung tissue were determined by Alkaline hydrolysis method. HE staining and Massons staining were performed to examine the pathological changes of lung tissues after bleomycintreatment. Results： On d14 after bleomycintreatment， the lung coefficients in wild type mice and AQP4-/- mice were 19fold （1269 ± 605 vs 680 ± 082，q=4204，P<005） and 23fold （1405 ± 582 vs 605± 058， q=5172，P<001） of that in control， respectively， but no significant difference was found between wild type and AQP4-/- mice in the lung coefficient value （P>005）. The hydroxyproline contents in the lung increased after bleomycintreatment; on d28， the lung hydroxyproline contents in wild type and in AQP4-/- mice were 155fold （085 ± 022 g/mg vs 055 ± 014 μg/mg， q=4313，P<005） and 14fold （084 ± 013 μg/mg vs 060 ± 014μg/mg， q=4595，P<005） of that in control， respectively， but no significant difference was noticed between wild type and AQP4-/- mice in lung hydroxyproline contents. There was a tendency that serum TGFβ1 and TNFα levels increased in bleomycintreated mice， but no significant difference was found between wild type and AQP4-/- mice. AQP4knockout showed no effects on pathological changes of lung tissues with HE staining and Massons staining in mice with bleomycininduced lung fibrosis. Conclusion： AQP4 might not be involved in bleomycininduced lung fibrosis in mice.