Abstract: Objective To study the significance of serum IgA/C3 ratio in the diagnosis of IgA nephropathy (IgAN) and differential diagnosis of other non-IgAN glomerular diseases, and the changes of IgA/C3 ratio in patients with IgAN before and after treatment. Methods The results of serum IgA, C3 and related laboratory tests from patients treated preliminarily in the Second Affiliated Hospital of Shantou University and the Sixth Affiliated Hospital of Sun Yat-sen University from January. 2013 to December.2017, whose were diagnosed with IgAN or primary non-IgAN or secondary glomerular diseases, were collected. The differences of the serum IgA/C3 ratio between IgAN and other related glomerular diseases and the IgA/C3 changes before and after treatment were analyzed. Results (1)The serum IgA/C3 ratio for IgAN,primary non-IgANand LN were 3.0±1.3, 1.9±0.8 and 6.9±4.9,respectively, with statistical significance of the differences of IgAN vs primary non-IgAN(P<0.01)and vs LN (P<0.01).(2) The serum IgA/C3 ratios for the primary non-IgAN subgroups minimal change nephritis(MCD), membranous nephropathy(MN) and focal segmental glomerulosclerosis(FSGS) were 1.9±0.8,1.9 ±0.8 and 2.1±0.5,respectively.There was statistically significant difference of IgAN vs MCD(P<0.01), vs MN (P<0.01) and vs FSGS IgAN (P<0.01).(3)By multivariate regression analysis, IgA/C3 ratio (OR=2.509,95%CI 1.196~5.262,P=0.015) could be regard as an important risk factor for patients to suffer IgA Ninthe primary glomerular disease group.(4) The serum IgA/C3 ratio sexhibited statistically significant difference of IgAN + dialysis vs non-IgAN +dialysis(3.7±1.8 vs (1.8±0.6),P=0.011 and vs non-IgAN + non-dialysis(3.7±1.8) vs (1.9±0.8),P=0.005.(5) In the ROC curve diagnosis for IgAN and primary non-IgAN diseases, the cut point of the serum IgA/C3 ratio was equal to or greater than 2.106 or greater with the area under the curve of 0.775 (P<0.01), which assisted to distinguish IgAN with other primary glomerular disease. (6) Among 43 IgAN patients who were followed up, 37 ones acquired remission in proteinuria 24 hours after treatment.The serum IgA/C3 ratios before treatment(with proteinuria remission) and after treatment (without proteinuria remission) were (3.1±1.1) vs (2.3±0.9),with difference of statistical significance(t=3.506,P=0.001).Conclusions Theserum IgA/C3 ratio is of significance in the diagnosis and differential diagnosis of IgAN among primary glomerular diseases, and the decrease of the ratio maybe a positive factor for prognosis of IgAN.