Abstract: ATRA has widely biological effects, which can regulate the proliferation, differentiation and apoptosis of cells. Therefore, the purpose of the experiment is to investigate the function of ATRA on the proliferation and differentiation of rat preadipocytes. In the present study, the effects of ATRA on proliferation and differentiation of preadipocytes was detected by MTT and quantitation of Oil Red O staining respectively; the morphologic changes of preadipocytes differentiation were analyzed by Oil Red O staining and Acridine Orange staining. The results of MTT show that the low concentrations （10^-8~10^-7 mol/L） of ATRA accelerate the proliferation of preadipocytes, but the high concentrations（10^-6~10^-4 mol/L）of ATRA inhibit it（P＜0.01）. The results of quantitation of Oil Red O staining show that the high concentrations（10^-6~10^-4 mol/L）of ATRA block the differentiation of preadipocytes, but the low concentrations（10^-8~10^-7 mol/L） of ATRA promote it（P＜0.01）. The staining pictures of Oil Red O and Acridine Orange demonstrate that when preadipocytes differentiate into adipocytes, they become oval and fill with lipid.