Abstract: Objective To explore the mechanism of sodium valproate （VPA） combined with granulocyte colony-stimulating factor（G-CSF） in the induction of differentiation of U937 leukemic cell line. Methods The U937 cells at logarithmic growth phase were treated with 0.5 mmol/L VPA or 50 ng/ml G-CSF alone and in combination. Cell differentiation was analyzed by Wright-Giemsa stain cell morphology and flow cytometry. Flow cytometry was performed to detect the expression of G-CSFR in the cells. The protein level of CCAAT/enhancer-binding proteins C/EBPα and G-CSFR were detected by Western blotting. Results VPA induced myeloid differentiation in U937 cells as demonstrated by up-regulation of CD11b from（11.49±3.48）% to （62.48±9.96）% （P<0.05）. When combined with G-CSF， the differentiation was remarkably enhanced to（97.65±0.49）% （P<0.05）. The expressions of G-CSFR（CD114） on U937 cells were also up-regulated from （29.50±6.80）% to （59.88±9.99）% （P<0.05）. VPA can enhance C/EBPα expression in these cells. The expression levels of G-CSFR protein were significantly up-regulated. Conclusion Combinations of VPA with G-CSF had an additive or synergistic effect on induction of myeloid differentiationin U937 cells. This effect may be mainly through the increase of C/EBPα and up-regulation of G-CSFR.