Aim This study was to seek the effect of oxidized low density lipoprotein(ox-LDL) on the activity of nitric oxide synthase(NOS)and releasing nitric oxide(NO)from culture endothelial cells in vitro. Methods The endothelial cells were exposed to different doses of ox-LDL from 0 to 200 mg/L and incubated (37℃、5% CO 2)for 24 hours, and the activity of LDH and NO amounts in the supernatant and the activity of NOS in endothelial cells were measured, and NOS positive staining cells were stained using NADPH-diaphorase (NADPH-d) histochemical method . Results Ox-LDL could increase the activity of LDH and decrease the NO amounts in the supernatant ,and the activity of NOS and numbers of NOS positive staining cell decrease in a dose effect manner. Conclusion All those suggested that ox-LDL could directly damage endothelial cell, and decrease the activity of NOS of endothelial cell, and NO releasing from endothelial cell. This may be one of mechanims that ox-LDL bring atherosclerosis.