Abstract: Objective To investigate the effect of p38 mitogen-activated protein kinase (MAPK) gene knockout on the proliferation of embryonic fibroblasts in mice (MEFs). Methods The expression of p38 in MEFs p38+/+ and p38-/- cells were detected by Western blotting. The growth curves of p38+/+ and p38-/- cells were plotted with the results of methylthiazoletetrazolium (MTT) eolorimetrie assay, and the ratios of different cell phases of p38+/+ and p38-/-eeUs were analyzed by flow eytometry. Results The growth curves showed that the growth rate was notably retarded and cell double time elongated in p38-/- cells, and there was 15.5% decrease of the number of p38-/- cells in comparison with that of p38+/+ cells in 96-hour culture.G2/M transition was inhibited in p38-/- cells. Meanwhile, G1/S transition was also inhibited in p38-/- cells, as shown by the results of flow eytometry. The ratios of G0/G1, G2/M, and S phases of p38+/+ cells were 34.47%, 10.81%, and 54.72%, respectively; while those of p38-/- cells were 48.49%, 4.06%, and 47. 44%, respectively. There were 40.7% increase and 13.3% decrease in the cell numbers of G1 and S phases of p38-/- cells in comparison with those of p38+/+ cells, respectively. Conclusion p38 gene knoekout in MEFs leads to cell cycle arrest and decreased cell proliferation.