The damage of capillary wall is the important cause of hemorrhage induced by radiation. Experiment showed previously that "8401" : 3-(3, 4, 5-trimethoxphenyl) -Propenoic acid ethylene diamine salt increased platelet number of the ir radiated mice. This paper observed the "8401" effect of different dose on influence of proliferation of the endothelial cells. The cultured cells of R,22-25, were irradiated with ,60,Co r-ray (15 GY) at room temperature, then the Cells with an initial density of 3.5×10,5, cells/ml were plated on twelve dishes and 1-5 µg of "8401" were added into two dishes each (exeastt tho dishes as control.) By the seventh days after platting cells, each dish were trypsmized and counted with a blood counter. The result was: 1 µg group obtainedg cells count of 1.86×10,5, cells/dish 2 ug group-2.17×10,5, 3 ug group-2.21×10,5, respectively, but control group is 1.23×10,5, cells/dish. The cells count in "8401" group was much higher than controlt (p,<,0.01). The results show that "8401" of a certain dose an stimulate growth of the irradiated endothelial cella and "8401" seem to promote proliferation of the endothelial cells in case of the endothelial cell injury.