Objective To investigate the regulation of melatonin (MT) on Toll-like receptor 4 (TLR4) signaling in diabetic db/db mice kidneys. Methods The 48 10-week-old male db/db mice were randomly divided into db/db group, db/db+MT 50 μg/kg group, db/db+MT 100 μg/kg group and db/db+MT 200 μg/kg group, each consisting of 12 mice. These mice received i.p. injections of MT These mice received i.p. injections of MT [dissoved in phosphate buffer solution (PBS)/ dimethylsulfoxide (DMSO) solution, given every day]. Alternatively, 12 db/m mice served as the control group. db/m and db/db group were injected i.p. with the same volume of PBS/DMSO solution. The animals were sacrificed after 12 weeks of dosage administration. Blood glucose (BG), body weight (BW), kidney weight (KW) and 24 h urinary albumin excretion rate (UAER) were determined; Kidney pathological lesions were evaluated by renal pathological staining. Immunohistochemistry of renal TLR4, NF-κB p65, and ED-1 was performed to determine the immunoreactivity. Western blotting was used to detect the expression of renal TLR4, myeloid differentiation factor 88 (MyD88), TIR-domain-containing adaptor inducing interferon-β (TRIF), interferon regulatory factor 3 (IRF-3) and NF-κB p65, while the mRNA expressions of renal tumor necrosis factor -α (TNF-α) and monocyte chemotactic protein-1 (MCP-1) were evaluated by real-time PCR. Results Compared with control group, the levels of BG, BW, KW and UAER were much higher in db/db mice group (P<0.01), while KW in db/db+MT (100, 200 μg/kg) groups and UAER level in db/db+MT (50, 100, 200 μg/kg) groups were distinctly decreased compared with those in db/db group (P<0.01). In week 12 db/db mice, the glomerular mesangial expansion index and tubulointerstitial injury index were increased compared with those in db/m mice (P<0.01). The above kidney histopathologic lesions were distinctly ameliorated by 50, 100, 200 μg/kg MT (P<0.05). Immunohistochemistry intensity of renal TLR4, NF-κB p65 and ED-1 displayed obvious differences between db/m mice and db/db mice (P<0.01), and that were remarkably decreased in db/db+MT (50, 100, 200 μg/kg) mice compared with db/db mice (P<0.05). Western blotting showed that the protein expression of renal TLR4, MyD88, TRIF, IRF-3 and NF-κB p65 were stronger in db/db group compared with those in db/m group (P<0.05) and weaker in db/db+MT (50, 100, 200 μg/kg) groups compared with those in db/db group (P<0.05). Futhermore, the mRNA expressions of renal MCP-1 and TNF-α were higher in db/db group compared with those in db/m group (P<0.01) and lower in db/db+MT (50, 100, 200 μg/kg) groups compared with those in db/db group (P<0.01). Conclusion Melatonin may partly down-regulate TLR4 signaling pathway to inhibit Inflammatory reaction and alleviate kidney injury in diabetic db/db mice.