Abstract: Objective To explore the serum expression level of miR-503 in patients with diabetic kidney disease(DKD) and examine its prognostic correlation. Methods From June 2017 to June 2020, 150 hospitalized DKD patients were selected as DKD group. And 116 patients with type 2 diabetes (DM) without kidney injury were selected as DM group and another 100 healthy subjects as control group during the same period. All of them were followed up for 6 months. Based upon whether or not there was end-stage renal disease(ESRD), they were divided into two groups of good prognosis and poor prognosis. Real-time fluorescent quantitative PCR(qRT-PCR) was utilized for detecting the serum expression level of miR-503 in DKD patients with varying severity of kidney injury. And the differences in serum miR-503 expression of patients with good prognosis and poor prognosis were compared. Univariate analysis was performed for examining the influencing factors of poor prognosis while multivariate Logistic regression for analyzing the risk factors for poor prognosis. Receiver operating characteristic (ROC) curve was employed for calculating the serum value of miR-503 in predicting poor prognosis. Results As compared with control group, the serum expression level of miR-503 was up-regulated in DKD/DM group and DKD group was significantly higher than DM group. The difference was statistically significant(P<0.05). The expression level of serum miR-503 was statistically significant in DKDpatients with different glomerular grades, interstitial fibrosis & tubular atrophy(IFTA), interstitial inflammation and clinical stages(P<0.05) and the level of miR-503 spiked with worsening kidney injury (P<0.05). The serum level of miR-503 was higher in poor prognosis group than that in good prognosis group(P<0.05). Univariate analysis indicated that the prognosis of DKD patients was not significantly correlated with age, gender, diabetic course, body mass index(BMI), fasting blood glucose(FBG) or HbAlc level. However, it was significantly correlated with 24 h urinary protein, creatinine(Scr), blood urea nitrogen(BUN) and estimated glomerular filtration rate(eGFR) (P>0.05Serum). Multivariate Logistic regression analysis revealed that Cr, eGFR and serum miR-503 level were independent risk factors for poor prognosis. ROC curve indicated that the area under the ROC curve(AUC) of serum miR-503 for predicting poor prognosis was 0.821 with a cut-off value of 4.834, a sensitivity of 84.20% and a specificity of 78.60%. Conclusion Serum miR-503 level is significantly up-regulated in DKD patients. Serum miR-503 has some diagnostic value in predicting the prognosis of DKD patients. Timely detection and proper interventions may improve the prognosis of patients.