Recent studies showed that low dose aspirin exerts a significant administration time-dependent effect on blood pressure in previously untreated hypertensive patients. However, this effect has not been confirmed in patients taking low-dose aspirin in combination with antihypertensive medications. A multi-center, double-blind randomized, cross-over, active-control, comparative clinical study will be conducted to evaluate the influence of aspirin on blood pressure in well-controlled hypertensive patients who receive low dose aspirin at different times of the day.