Objective To explore the possible relationship between alteration of cell cycle and JAK/STAT3 signal transduction pathway inhibition induced by arsenic trioxide (As_2O_3,) in myeloma cell lines U266 and RPMI8226 in vitro. Methods Multiple myeloma cell lines U266 and RPMI8226 were used as in vitro models. The influence of AS_2O_3 on myeloma cells were evaluated by MTT assay and flow cytometry. Meanwhile, methylation specific PCR and Western blotting were employed to detect the methylation status of gene SOCS-1 and protein expression level of P-STAT3 in these cells after AS_2O_3 treatment. Results AS_2O_3 significantly inhibited the growth of U266 and RPMI8226 cells in a dose-dependent manner. Furthermore, cell cycle was arrested at G0/G1 phase with inhibition of protein expression level of P-STAT3 and SOCS-1 gene demethylation after exposure to As_2O_3 for 72 h( r = 0. 85, P < 0.05). Conclusion AS_2O_3 could induce the alteration of cell cycle which might be related to JAK/STAT3 signal transduction pathway inhibition and SOCS-1 demethylation in myeloma cell lines. The study puts forward a new idea of AS_2 O_3 treatment in multiple myeloma.