Cyperus rotundus L. (Cyperaceae) has been used as an analgesic, antiinflammatoty agent, diuretic and emmenagoga in folk remedies. Cyperi Rhizomata, processed and unproces sed, were extracted with MeOH and fractionated with petroleum ether (Pet. Ether), $CHCl_3$, BuOH, water. In order to investigate the effects of their fractions on kidney function of acute renal failure rats induced by $HgCl_2$ urinary volume, BUN, Creatinine, Uric acid were determined. The diuretic effect of processed Cyperi Rhizoma was significantly increased in renal failure rats, on serum chemical parameters, the significant inhibition of BUN (blood urea nitrogen) of processed Cyperi Rhizoma was revealed.