The aim of this study was to explore the effect of 2 different spliceosomes of X-box binding protein 1 (XBP-1), the spliced form XBP-1s and unspliced form XBP-1u, on myeloma cell differentiation and its mechanism. The overexpression plasmids pcDNA3.1-C-XBP1u and pcDNA3.1-C-XBP1s were constructed and transfected into myeloma cell line U266, RPMI8226. The morphology of U266 and RPMI 8226 cells was observed by means of light microscope, the expression rate of CD49e on cell surface was detected by flow cytometry, the ELISA was used to determine the changes of light chain protein level in supernatants of cell culture, the Western blot was used to assay the expression changes of XBP1u and XBP1s. The results showed that the overexpression of XBP1u could promote the myeloma cell differentiation morphologically displaying the maturation of plasmocytes, the CD49e positive expression rates on surface of U266 and RPMI8226 cells were obviously up-regulated from 9.02±0.3% and 5.17±0.92% in control group to 27.7±1.14% and 13.97±1.79% respectively (p<0.01), the levels of light chain protein in supernatants of U266 and RPMI 8226 cell cultures increased from 474.75±19.52 ng/ml and 289.44±6.19 ng/ml in control group to 692.34±21.17 ng/ml and 401.55±13.7 ng/ml respectively (p<0.01, p<0.05), while the above-mentioned parameters in the overexpressed XBP-1s showed no significant changes, which indicated no promotive effect of overexpressed XBP1s on myeloma cell differentiation. It is concluded that the up-regulation of XBP-1u expression plays an important role in the differentiation of myeloma cells.