Abstract： Objective To investigate the possible mechanism of arsenic trioxide (As2O3) inducing P16 gene demethylation and transcription regulation in the retinoblastoma (RB) Cell Line Y79. Methods The induced growth inhibition of Y79 cell was assayed by MTT;The DNA content of Y79 cell was analyzed by flow cytometry after being exposed to As2O3;the methylation status of the P16 gene in Y79 cell line before and after treatment with As2O3 was detected by the nested-methylation specific PeR and DNA sequencing;the mRNA of P16, DNA methyltransferases (DNMT3A and 3B)gene were determined by RT-PCR. Results As2O3 was able to inhibit the growth of Y79 cell and increase the cell number in G0-G1phase;P16 gene was not expressed in Y79 cell line and As2O3 can induce it's mRNA expression;after 48-hour disposal of As2O3,the methylation levelof P16 gene was apparently attenuated in Y79 cell line, the expression of DNMT3A and DNMT3B was obviously down-regulated. Conclusions P16 gene is the hypermethylation in the retinoblastoma cell line Y79, and As2O3 can inhibite the methylation of P16 gene and up-regulate the expression of p16 gene mRNA which inhibits the proliferation of Y79 cell by inducing the G0-G1arrest, by inhibiting the expression of DNA methyltransferases.