Abstract: It is known that the function of endothelial cells is associated with the presence of many diseases. However, so far the quantitative descriptions of their function have been seldom investigated. In this paper, the function of endothelial cells is evaluated by the elastic modulus. The theoretical calculation for detecting their Young's modulus based on indentation theory is established. The Young's moduli of the normal endothelial cells and the endothelial cells after hydrogen peroxide treatment are investigated by the scanning probe microscopy. It is found that after hydrogen peroxide treatment the Young's modulus of the endothelial cells will be increased. Thus, it is proved that the activity of the cell can be characterized by its elasticity. Finally, errors in the measurement are discussed.