Functioning nephron mass (number of nephrons in grafted kidney) is one of the nonimmunologic factors that may have some impact on long- term graft survival. The aim of this study was Impact of Donor Nephron mass on recipient graft outcome. During 1989 – 2005, 1000 renal transplants was carried out at our center. 217 of them were studied and followed for an average of 8 years. All patients received grafts from living donors. Weight of grafted kidney (donor nephron mass) and also recipient BMI were measured at the time of operation in all cases. Nephron mass index (NMI) was defined as the ratio of donors’s nephron mass to recipient’s BMI. This ratio was calculated for all 217 cases. Associations between variables was tested by logistic regression and pearson correlation. The analysis was performed with the SAS system and Splus statistical software. To evaluate the graft function, serum creatinine, acute and chronic rejection episodes were determined. Mean NMI was 8.07 (±0.2) and mean creatinine level was 1.43 (±0.4) mg/dL. There were 32 cases (14.7%) of acute rejection, managed successfully with antithymocyte globulin (ATG) in 28 cases. Four patients lost their graft. There were five cases of graft loss due to chronic rejection. Using Pearson correlation, we found no association between NMI and mean serum creatinine level. Logistic regression showed significant relation between NMI and acute rejection (P<.05), with odds ratio of 2.0. There was no significant correlation between NMI and chronic rejection. The less NMI, The higher acute rejection. However, in long term, no significant correlation between graft survival and NMI was found. Also, mean creatinine level was not significantly different in patients regardless of their NMI was found.