Abstract: Objective To investigate the impact of overexpression of RbAp46 on Proliferation and Differentiation of monoblastic cell line U937. Methods Electroporation was performed to establish U937 cells stably overexperssing RbAp46 gene. Viability of transfected cells was assayed by trypan blue exclusion. Cell number was counted daily to measure the growth rate. Cells at an initial concentration of 2×105 cells/ml were incubated with or without 50 ng/ml TPA. After 72h,cell surface antigen CD11b,mRNA p21WAF1/CIP1 and cell cycle distribution were determined by flow cytometry, RT-PCR,and flow cytometry, respectively. Results The U937 cells expressing exogenous RbAp46 showed about 1-fold lower growth rate than control cells without exogenous. The expression of CD11b in RbAp46-transfected U937 cell line was upregulated regardless the presence of TPA. Furthermore, the expression level of p21WAF1/CIP1 in RbAp46 transfected cells was comparable to that in control cells at mRNA level, while U937/RbAp46 cells treated with TPA had a marked increase in p21WAF1/CIP1 mRNA expression and a G1 phase arrest. Conclusion Overexpression of RbAp46 inhibited the proliferation of U937 and triggered several differentiation programs in which p21WAF1/CIP1 may be involved.