Background: Management of diabetes mellitus induced-erectile dysfunction (DMED) is challenging because of its poor responses to phosphodiesterase type 5 inhibitors. Increasingly important roles of 12-lipoxygenase (12-LOX) have been proven in diabetes mellitus. Aim: To investigate 12-LOX activity and therapeutic effect of its inhibitor, baicalein (BE), on DMED. Methods: Intraperitoneal streptozotocin injection was used to induce type I DM, and an apomorphine test was used to evaluate erectile function. In experiment A, we assessed 12-LOX expression alteration in the corpus cavernosum (CC) of rats with DMED of different levels of severity. In experiment B, rats with DMED were intraperitoneally injected with BE for 4 weeks, and control rats were injected with vehicles. The erectile function was tested by cavernous nerve stimulation before penile tissue was harvested. We performed Western blot, immunohistochemistry, immunofluorescence, Masson trichrome staining, and enzyme-linked immunosorbent assays to measure related proteins in CC. Main Outcome Measure: The main outcome measures included rectile response, histologic examination, and expression alteration of related proteins. Results: 12-LOX upregulation was associated with the progression of type I DMED. After 4 weeks treatment, compared with the DMED group, the DMED thorn BE group showed better erectile responses to cavernous nerve stimulation. In theDMEDthornBE group, significantly enhanced endothelial nitric oxide synthase/nitric oxide/cyclic guanosine monophosphate pathway, reduced 12-LOX expression, and inhibited p38 mitogen-activated protein kinase/arginase II/L-arginine pathway were showed in CC relative to the DMED group. In addition, overactivated oxidative stress and fibrosis in the DMED group were both partially ameliorated in the DMED thorn BE group. Clinical Implications: BE may be considered as an effective therapy for DMED, but needs to be verified in future human investigations. Strengths & Limitations: The role of 12-LOX and its inhibitor, BE, is firstly demonstrated in rats with type I DMED. However, the experimental data are derived from animal models with without evidences from cellularbased experiments. Conclusion: 12-LOX might serve as an important factor in the pathogenesis of type I DMED. BE alleviated erectile dysfunction in rats with type I DMED probably by inhibiting 12-LOX expression, ameliorating endothelial nitric oxide synthase dysfunction, as well as suppressing oxidative stress and fibrosis.