The mammalian target of rapamycin (mTOR) is relevant to cell senescence and organismal aging. This study firstly showed that the level of mTOR expression increased with aging in rat kidneys, rat mesangial cells and WI-38 cells (P < 0.05). The levels of phosphorylated-mTOR (p-mTOR), cyclin D1 and p21(WAF1/CIP1/SDI1) expression were significantly higher in WI-38 cells treated with l-leucine (an activator of mTOR) (P < 0.05). The positive staining ratio of senescence-associated beta-galactosidase, number of cells in G1 phase, and cellular volume were all increased in WI-38 cells treated with l-leucine when the cellular population doubling (PD) number was 34, while the above phenotypes did not appear in control group until its PD number reached 40 (P < 0.05). The levels of p-mTOR, cyclin D1, and p21(WAF1/CIP1/SDI1) as well as the aging-related phenotypes were all reduced in cells treated with rapamycin (an inhibitor of mTOR) than in control cells (P < 0.05). These results demonstrated that the level of mTOR was increased in kidney with aging, and that mTOR may promote cellular senescence by regulating the cell cycle through p21(WAF1/CIP1/SDI1), which might provide a new target for preventing renal aging.