Background/Aims: Although numerous studies have explored the mechanisms regulating the enzyme activity of NADPH oxidase in diabetic nephropathy (DN), little information is available for the contribution of microRNAs (miRNAs) to the regulation of NADPH oxidase expression. Therefore, the present study was to test whether miRNAs importantly contribute to the regulation of NOX4 expression, a major catalytic subunit of NADPH oxidase under hyperglycemia. Methods: Diabetic rats were induced by streptozotocin. miRNA microarray, Western blot, real-time RT-PCR and luciferase reporter assays were employed in this study. Results: Among 5 miRNAs, which are predicted to have a binding capacity to rat NOX4, the miRNA-25 level was significantly reduced both in the kidney from diabetic rats and in high glucose-treated mesangial cells, accompanied by the increases in NOX4 expression levels. In an in vitrostudy, we found that NADPH activity was increased by 226.2% in miRNA-25 inhibitor transfected cells and decreased by 51.0% in miRNA-25 precursor transfected cells. miR-25 inhibitor dramatically increased both NOX4 mRNA and protein levels. We then showed that miR-25 negatively regulated NOX4 expression by directly targeting the 3′-UTR by luciferase reporter assays. It was found that transfection of miR-25 precursor significantly decreased the luciferase activity of NOX4 3′-UTR by 39.5%, whereas the mutant sequence restored levels to 79.4%. Finally, our results indicated that the miR-25-mediated NOX4 mRNA level may result from the regulation of mRNA stability. Conclusions: These findings for the first time indicate that miRNA-25 may serve as an endogenous gene silencing factor and contributes to the regulation of NOX4 expression and function in DN.