Purpose: Aortic aneurysm (AA) and aortic dissection (AD) are common diseases among the elderly people and potentially lethal. However, there is currently no management to prevent these diseases. Quercetin, an abundant polyphenol in onions, is reported to improve vascular function. In this study, we investigated the effects of quercetin on AAs or AD in mice. Methods: In C57BL/6J male mice, angiotensin II (Ang II) and β-aminopropionitrile (BAPN) were administered to induce hypertension and degeneration of elastic lamina respectively, leading to AAs. If L-NAME were added, mice shows endothelial dysfunction and more AD incidence. We call these AA model as "AB" and AD model as "LAB". Results: Quercetin treatment reduced the incidence of AAs and the death from aortic rupture in AB mice, and the incidence of AD and rupture in LAB mice. The activity of MMP-2/9 were upregulated in aortae, but suppressed by quercetin treatment. The expressions of vascular endothelial cell adhesion molecule-1 and F4/80, a marker of macrophages, are also suppressed in quercetin treated group. Conclusions: These findings suggest that quercetin prevents AA and AD via its protective effects against endothelial dysfunction, elastin degeneration and inflammation.