Objective:To study the PDT effects on culturing endothelial cell in vitro under the differens content of CPD 4. Methods: According to the existing techniques obtaining prime umbilical vein endothelial cell in 24 well culturing plate,10 5/well.Add 1640 medium mixed with different final concentration of CPD 4 individuallly and incubate for 1h. Then shine them with 630～640 nm red light. Observe the morphological changes of EC. After PDT 6 h,measure the contents of Prostaglandin F 1?(PGF 1?) and and LDH in the culturing medium and the percentage of the residual cell. Results:With the concentration of CPD 4 increasing, the damage degree of EC aggravates. There was emergence of EC contracting ,falling off , cytoplasmic swelling,floating and dying. The contents of PGF 1? and LDH were increasing and the percentage of residual cell decreases simultaneously. There was statistical significance between the five groups. Conclusion:The effect of PDT is direct.The effectiveness has two characteristic: ①CPD 4 induced PDT has direct photosensitised damage to EC. There exists CPD 4 dose dependence.②The PDT target of EC is membrane and cytoplasma organelle. The damage is lethal and irreversible.