The underlying pathology of most cardiovascular diseases (CVDs) such as coronary artery disease, high blood pressure, and stroke involves decreased cardiovascular contractility and anatomic alterations in cardiovascular structures. Nitric oxide (NO) regulates vascular tone and contractile function of myocardium and maintains blood vessel homeostasis. Interestingly, the effect of NO is like a double-edged sword in the body. Insufficient NO causes hypertension and atherosclerosis, while an overproduction of NO may foster inflammation and cause heart infarction and shock. In addition, growing evidences have shown that oxidative stress plays pivotal roles in the initiation and progression of CVDs. This chapter will discuss in detail the roles NO plays in the cardiovascular system under both physiological and pathological conditions. We will focus on: (1) the molecular mechanism of cardiovascular contraction, (2) NO/Ca 2+-induced muscle relaxation, (3) NO-related structural change in blood vessels, and (4) redox balance in the cardiovascular system. The relationships between these molecular mechanisms and the characteristics of CVDs will be highlighted.