To investigate the oxidized low density lipoprotein (oxLDL) on U937 cell ATP-binding cassette transporter A1 (ABCA1) mRNA expression and cholesterol efflux situation. Human U937 cells were incubated with gradient concentrations of oxLDL(0, 25, 50, 75, 100, 125mg/L), and then dyed by oil red O to estimate the content of intracelluar lipid and detect the expressing quantity of ABCA1 mRNA by Real-time Fluorescence quantitative PCR simultaneously. Calculating the cholesterol efflux rates by using the scintillation counter to detect the amount of H3-cholesterol in each well cell culture plate and medium. Real-time Fluorescence quantitative PCR analysis showed that the expression levels of ABCA1 mRNA in monocytes were lower than basal line when not intervened with oxLDL, and increased drastically with oxLDL stimulation, significant difference compared with controls(P<;0.01), and reached the highest level at oxLDL 50mg/L, nevertheless, continously increasing the concentration of oxLDL above 50mg/L, the expression decreased. So is the outflowing rate of intracelluar lipid. Oil red O dyeing results also suggested that celluar lipid content was the highest when intervened with 125mg/L oxLDL, and increased most obviously at 50mg/L oxLDL. Cholesterol outflow result also demonstrated that cholesterol outflow rate related with the ABCA1 mRNA expressing quantity. With the increase of intervening concentration of oxLDL on U937cells, the exprssion of ABCAl mRNA represented that rising befor 50mg/L oxLDL, and then decreasing, reaching the top point at 50mg/L oxLDL. so was the change in the outflowing rate of intracelluar lipid.