AIM: To explore the correlation between trimethylamine-N-oxide (TMAO) and type 2 diabetic kidney disease (DKD), and to provide new ideas for the early clinical diagnosis of DKD. METHODS: A total 246 patients with type 2 diabetes mellitus (T2DM) admitted to the Department of Endocrinology of the First Hospital of Lanzhou University from January 1, 2020 to May 31, 2020 were divided into diabetic kidney disease group (DKD group) and simple diabetes mellitus group (NDKD group). According to urinary albumin/creatinine ratio (UACR), the patients were divided into A1, A2 and A3 subgroups. According to the estimated glomerular filtration rate (eGFR), the patients were divided into G1, G2, G3 and G4-5 subgroups. According to the Kidney Disease: Improving Global Outcomes (KDIGO) guidelines, the risk of progression of DKD was assessed (low, medium, high or very high risk). General clinical data and laboratory indicators were collected. TMAO level was measured by euzymelinked immunosorbent assay. SPSS 25.0 software was used for statistical analysis. RESULTS: In T2DM patients, TMAO level was positively correlated with UACR (r=0.515, P<0.01) and negatively correlated with eGFR (r=-0.409, P<0.01). TMAO is an independent risk factor for the onset and progression of DKD. In diagnostic model, the AUROC was 0.745 with optimal cut-off value was 5.37 μmol/L. CONCLUSION: TMAO is closely related to the occurrence and development of DKD, and it has certain clinical predictive value for DKD. Therefore, TMAO may become a potential target for the early diagnosis and treatment of DKD.