Aim To investigate the effects of CD40 ligand from activated platelets on the expression of tissue factor and tissue factor pathway inhibitor in human vascular endothelial cells(hUVEC) and its molecular mechanism. Methods Activated platelets and human umbilical vein endothelial cells were co-incubated in the same systerm,and then the expression of tissue factor and its inhibitor in endothelial cells and the effects of blocking CD40-CD40L signal pathway on their expression were observed. Result After 8 hours of the co-incubation of activated platelets and human umbilical vein endothelial cells,tissue factor mRNA and its protein expression level in hUVEC were raised.Nevertheless these effects could be attenuated by CD40L monoclonal antibody and CD40 monoclonal antibody added into media before platelets being induced(P<0.05),but tissue factor pathway inhibitor mRNA expression level in hUVEC was not changed. Conclusions Activated platelets could induce the expression of tissue factor in endothelial cells by the expression of CD40L, but could not enhance the expression of tissue factor pathway inhibitor.This indicates that these effects may play a promoting role in unstable plaque formation and the development of atherosclerosis.