Abstract： Objective To explore the role of the change of the CD11b expression in hyperleukocytosis and acute promyelocytic leukemia(APL) differentiation syndrome, we observed the effects of all-trans retinoie acid(ATRA) and trioxide arsenic(ATO) alone or the combination together with daunorubicin (DNR) on CD11b expression of human APL cell line NB4 cells. Methods By using the mono-antibody of CD11b FITC-conjugated, we monitored the level of CD11b expression of NB4 cells by flow cytometry. Results With the length of the 1 μmol/L ATRA exposure,the CD11b expression was increased. Exposed to ATRA for 24 h, 48 h, 72 h and 168 h;the CD11b expression of NB4 cell was (33.34±3.15)%, (55.59±5.13)%, (86.08±5.12)% and (90.69±2.69)%, respectively which were higher than control group and the difference was statistically significant (P<0.01). With the length of the 1 μmol/L ATO exposure, the CD11b expression showed no significant difference with control group(P>0.05), but was significantly lower than that in ATRA exposure(P<0.01). Exposed to 1 μmol/L ATRA combined with 1 μmol/L ATO for 72 h and 168 h, the CD11b expression were (18.81±1.40)% and (25.61±4.54)%, which were significantly higher than that in control group (P<0.05, P<0.01), but significantly lower than ATRA exposure (P<0.01). With the exposure to 1 μmol/L ATRA combined with 1 μmol/L ATO and 1 μmol/L DNR, the CD11b expression was also significantly lower than ATRA exposure (P<0.01) and showed no difference with control group (P>0.05). Conclusion By avoiding the increase of CD11b expression of promyelocytic leukemia cells, ATRA combined with ATO or two drugs together with DNR could play a role in eliminating incidence of hyperleukocytosis and retinoic acid syndrome.