To study effects of arachidonic acid (AA) on the growth and differentiation of rat adipocytes, a cells culture system of rat primary preadipocytes was established. The cells treated by different concentration of AA supplemented based on DMEM medium containing 10% fetal bovine serum. Cell proliferation was measured by trypan blue exclusion and methyl thiazolyl tetrazolium (MTT) assay method. Hoechst33342 fluorescence staining observed AA induced morphological changes. Oil Red O staining extraction assay assess the degree of adipogenesis and differentiation, and cyclooxygenases-2(COX-2) mRNA were detected by reverse transcriptase-polymerase chain reaction (RT-PCR). The results showed that rat preadipocytes treated with 120 μmol/L AA for 24-72 hours remarkably promoted the cells proliferation compared with control, 160 μmol/L AA treated for 48 hours could induce apoptosis of preadipocytes. 40, 80 μmol/L AA decreased the fat content in cells at 72 hours, and 40 μmol/L AA significantly up-regulated the expression of COX-2 mRNA at 24 hours. This results indicate that AA regulate adipocytes proliferation and differentiation depended on treatment time and concentration. 40-80 μmol/L AA maybe useful to control body fat, which may be associated with the increase of COX-2 mRNA.