Background: Membranous nephropathy (MN) is among the most common causes of nephrotic syndrome in adults. MN is diagnosed in one third of cases of nephrotic syndrome on kidney biopsy. Kidney biopsy is the gold standard for diagnosing MN and plays an important role in determining the severity of the disease and in determining treatment decisions and regimens. Therefore, the lack of research on kidney biopsy in Mongolia is the reason for this study. Aim: The aim of this study was to investigate the pathological features in the kidney tissues of patients with primary membranous nephropathy diagnosed by kidney biopsy. Materials and Methods: A retrospective study was conducted on 51 cases of MN diagnosed in kidney biopsies performed at the First Central Hospital of Mongolia (FCHM) over a period of 12 years. Renal function was calculated using the CKD-EPI (2021) formula and classified into the stage of CKD by eGFR. Histopathological findings were examined using 4 light microscopy (LM) stains (Hematoxylin-Eosin, Masson-Trichrome, PAS, and Methenamine silver staining) and 8 immunofluorescence (IF) microscopy stains (IgG, A, M, complement C3, C4, C1q, and kappa, lambda). The study excluded secondary MN based on viral markers, tumor markers, and serological tests. Statistical analysis was performed using SPSS and STATA 15.0 software, using t-tests, Pearson’s chi-square tests, and multiple group comparisons were performed using ANOVA and Kruskal-Wallis methods. The study design was approved by the Ethics Committee of the MNUMS, Mongolia. (№ 2023/3-07) Results: A total of 305 kidney biopsies performed at the Kidney Center of the FCHM between 2011 and 2023 resulted in the diagnosis of 51 cases of primary MN. The mean age of patients with membranous nephropathy was 40.6±9.3 years, with the oldest age of 65 and the youngest of 22 years, and 36 (70.59%) were male and 15 (29.41%) were female. In the kidney biopsy, the average number of glomeruli was 16.51±7.82 (min-max, 3-54), and by LM, 33.3% showed global sclerosis of glomeruli by hematoxylin-eosin staining, 94.12% showed thickening of the glomerular basement membrane (GBM), 31.2% showed double counter staining of subepithelial immune complexes by methenamine-silver staining, 88.24% showed holes in the GBM, and 54.9% showed spike-like changes by Masson-Trichrome staining. IF showed IgG 3+ in 37.3%, 2+ in 39.2%, 1+ in 13.7%, and trace staining in 9.8%, while 74.5% of the cases were positive for C3, 93.1% for kappa, and 79.5% for lambda. LM showed thickening of the GBM (OR 23.5, 95% CI 0.093-0.53, p value= 0.007) and interstitial fibrosis (95% CI 6.98-31.07, p value= 0.003) contributing to the decrease in eGFR. The mean time from the onset of the first symptoms of kidney disease to the time of kidney biopsy was 35.35±61.54 months. Patients who underwent biopsy later (in months) after the diagnosis of the disease had a higher incidence of interstitial fibrosis (74.6 ± 98.43, 95% CI -90.52-20.68, p value = 0.002). Conclusion The histopathological features of MN confirmed by kidney biopsy showed thickening of the GBM in 94.12%, global sclerosis in 33.3%, and holes in 88.2%. Immunofluorescence microscopy showed 100% IgG staining, while C3, kappa, and lambda were positive in 74.5%, 93.1%, and 79.5%, respectively.