Aim:  The aim of this study was to assess the biological effects of oxidative stress on human vascular endothelial cells. Methods:  The telomeric changes and the alterations of the expression of telomere‐associated proteins in human umbilical venous endothelial cells (HUVEC) cultured in the presence of hydrogen peroxide (H2O2) were analyzed. Results:  During the culture, the cell growth rate decreased, whereas the telomerase activity of the surviving cells increased. As the H2O2 level increased, long telomeres decreased proportionally, thus resulting in a telomere length distribution that was rich in short telomeres. These observations suggested that H2O2‐affected endothelial cells bear telomeric features similar to those of aged cells. In contrast, the expression of telomere‐associated proteins, TRF1 and TRF2, showed different changes. TRF1 increased in relation to H2O2 concentration, whereas TRF2 showed no significant change. The surviving cells exposed to H2O2 showed a H2O2‐dose dependent increase in telomerase activity, whereas the telomere protein and RNA components were only elevated in low concentrations of H2O2. Conclusions:  The increase in telomerase activity and TRF1 protein expression of vascular endothelial cell might show an aspect of cellular protective reaction against oxygen stress. Geriatr Gerontol Int 2013; 13: 774–782.