This study was purposed to investigate the effect of bortezomib (Bor) and arsenic trioxide (As(2)O(3)) combination on multiple myeloma cell line KM3 and its mechanisms. KM3 cells were cultured with different concentration of Bor or As(2)O(3) as well as both for a certain time. The cell proliferation was analysed by MTT assay and the concentration of 50% proliferation inhibition (IC(50)) was calculated. Early apoptosis and late apoptosis of KM3 cells were detected by Annexin-V-FITC Kit, and the change of transmembrane potential was measured by flow cytometry. mRNA of Caspase-3, Bim and Bcl-xL were detected by RT-PCR. The results showed that the proliferation inhibitory rate of KM3 cells treated by Bor plus As(2)O(3) was much higher than that of KM3 cells treated by Bor only for 72 h [ (27.64 ± 0.81)% vs (21.67 ± 2.20)%, P < 0.05]. There were more KM3 cells treated by Bor plus As(2)O(3) in early apoptosis at 48 h and late apoptosis at 72 h than that of KM3 cells treated only by Bor [ (53.20 ± 3.70)% vs (35.40 ± 2.58)%, P < 0.01; (63.96 ± 2.97)% vs (54.08 ± 3.76)%, P < 0.01]. Transmembrane potential (Δψm) of KM3 cells treated by Bor plus As(2)O(3) decreased more at 48 h, as compared with Bor alone. The expression levels of caspase-3 mRNA and Bim mRNA in KM3 cells treated with Bor plus As(2)O(3) were higher than that in KM3 cells treated with Bor alone. But the expression level of Bcl-xL mRNA was lower than that in KM3 cells treated with Bor alone. It is concluded that As(2)O(3) can enhance the apoptosis-inducing effect of Bor on multiple myeloma cell line KM3, which is associated with decreasing the expression of Bcl-xl mRNA and increasing the expression of Caspase-3 and Bim mRNA.