In this study, it was aimed to investigate that whether any change in activity of catalase (CAT), superoxide dismutase (SOD), glutathione peroxidase (GSH-Px), lipid peroksidase and nitric oxide (NO) over time, and whether any difference between the different doses by giving different doses of aspirin to endothelial cells. Endothelial cells (HUVEC) in 24 wells microplates used in this study and 25, 50, 100, 250, 500, 750, 1000 and 1500 μM aspirin to 2 of 4 wells at each row were given, the other 2 wells were included as controls. Accordingly, while CAT, SOD, GSH-Px levels and lipid peroxidation were being measured, NO release from cell media was observed. The significant differences were not found between the baseline (0 hour) CAT, SOD, GSH-Px and lipid peroxidase levels measured from lisates that obtained from the cells that different drug doses given and controls (p>0.05). Also, CAT, SOD, GSH-Px and lipid peroxidase levels at 24 (p>0.05), 48 and 72 hours did not show any difference among different drug doses and control (p>0.05). In the control group significant differences were found between CAT, SOD and GSH-Px levels measured at 0, 24, 48, 72 hours (p<0.05, p<0.05 and p<0.05 respectively) but lipid peroxidase activity and NO levels showed no difference. Increase in antioxidant enzyme activity in the cells that aspirin was not given, caused by raised free radical formation due to increase in number of cells by time was observed. Aspirin prevented the increase in reactive enzyme activity which increases by time. These results suggest that nontoxic doses of aspirin might protect the cells.