Abstract： Objective To document the impact of conversion to mycophenolate mofetil (MMF)at different time points after transplantation on the renal function of renal function.Methods A longterm,multicenter,non-interventional and observational study was done.Two cohorts were included:One was Switch cohort (340 cases) including renal allograft recipients who switched to MMF at least 6 months after renal transplantation and followed up for 4 years after switch; The other was Stay cohort (123 cases),including renal allograft recipients who received MMF treatment after transplantation and followed up for 4 years after enrollment.Results GFR values of patients in Switch cohort was significantly increased after switch,and the change in GFR slope was 3.1 mL· min-1 · year-1 (P＜0.01).GFR values of patients in Stay cohort kept steady before and after enrollment,and the change in GFR slope was 0.44 mL·min-1 ·year-1 (P＞0.05).Statistically significant difference in the onset time of GFR decline (defined as 20％ decline from the baseline) was observed among subgroups within Switch cohort (P＜0.01),but there was no significant difference among subgroups within Stay cohort (P＞0.05).Stay cohort was 12％ higher than in Switch cohort every year.Conclusion Conversion to MMF ＞6 months or even many years after transplantation can obviously improve the renal function of recipients.The earlier conversion can benefit improvement of the renal function.