In order to study the activity and gene expression of DNA methyltransferase (DNMT) on U266 myeloma cells and to analyze their significance, the activity of DNMT was detected by ELISA, and the expressions of DNMT1, DNMT3a and 3b were analyzed by RT-PCR. U266 cells were treated by phenylhexyl isothiocyanate (PHI), and the change of activity and gene expression of DNMT were determined. The results indicated that the activity and expression of DNMT in U266 myeloma cells were higher, compared with normal control. After being treated by different concentration of PHI, U266 cells were driven into apoptosis and the activity of DNMT decreased obviously and the mRNA level of DNMT declined. It is concluded that the activity and gene expression of DNMT on U266 myeloma cells are higher, and DNMT may be a new therapeutic target of multiple myeloma.