The aim of this study was to observe the effect of p38MAPK inhibitor SB203580 on ATRA-induced differentiation of NB4 cells. The proliferation activity of cells was assayed by MTT method, the cell cycle was detected by flow cytometry, the differentiation of NB4 cells into granulocytes was measured by test of NBT reduction, the activity of extracellular signal-regulated kinase (ERK) was detected by substrate phosphorylation. The results showed that the ATRA in 0.01-01 micromol/L inhibited the proliferation of NB4 cells in time-and dose-dependent manner and induced the differentiation of NB4 cells into myeloid; the ATRA stimulated ERK activity in this process; ERK inhibitor PD98059 could partially block ATRA effect, specific inhibitor of p38MAPK, SB203580, combined with ATRA also could partially block the effects of ATRA on inhibition of NB4 growth and induction of differentiation. It is concluded that the ATRA stimulates ERK and p38MAPK pathway in the process inducing differentiation of NB4 cells, the ERK and P38MAPK may be necessary for the ATRA-induced differentiation in NB4 cells.