Abstract: Objective：To observe the effects of all-trans-retinoic acid（ATRA） on the expression of RARβ and p21 gene and also proliferation of thyroid squamous cell carcinoma cell line (SW579)so as to explore its mechanism. Methods ：The cells were randomly divided into control group and ATRA groups.ATRA groups were treated with different dose of ATRA respectively (final concentration of 10-7 mol/L、5×10-7 mol/L、10-6 mol/L、5×10-6 mol/L、10-5mol/L).24 hours later,ATRA at different concertrations were added.Treated with ATRA for 48 hours,the growth and shape of the cells were observed with a convert microscope.The proliferation of the cells was observed with MTT and the cell cycle was measured by flow cytometry;The mRNA expression levels of RARβ and p21 were determined by RT-PCR. Results ：After cells were treated with ATRA at low concentration respectively，the malignant phenotypes of cells gradually changed to the mature cells.At high concentration groups cells changed unremarkably.The proliferation index decreased markedly with a dose-dependent manner.Flow cytometry analysis showed the percent of G1 phase cells was significantly higher than that of the control groups and the percent of S phase cells was lower than that of the control groups.The cell cycle was arrested at G1 stage.The expression of the RARβ and p21 mRNA increases dramatically with ATRA at different concentrations. Conclusion ：ATRA can inhibit the proliferation of thyroid squamous cell carcinoma cell line with a dose-dependent manner, and the mechanism of which may be related to increases of RARβ and p21 mRNA expression.