Increased prevalence of obesity and impaired glycemic control is a major challenge in the western society. Clinical studies have indicated that obesity is an independent predictor of cardiovascular risk. Metabolic syndrome, a cluster of hyperglycemia, hypertension, excess body fat and abnormal cholesterol, is associated with impaired vascular functions such as endothelial dysfunction and arterial stiffness. The mechanism leading to these vascular abnormalities is not well understood. In our current study, we fed C57BL/6J mice with a high fat/high sucrose (HFHS) diet for 6 months with or without treatment with the arginase inhibitor ABH (2-(S)-amino-6-boronohexanoic acid, 10 mg/kg/day) in drinking water. Elevated activity of arginase (ARG), an enzyme implicated in many cardiovascular diseases, can compete with nitric oxide (NO) synthase for their common substrate, L-arginine. This leads to reduced bioavailability on NO and endothelial dysfunction. Increased arginase activity also provides more ornithine for synthesis of polyamines via ornithine decarboxylase (ODC) and proline/collagen via ornithine aminotransferase (OAT), leading to vascular cell proliferation and collagen formation, respectively. We hypothesized that elevated arginase activity is involved in vascular dysfunction and arterial fibrosis/stiffness associated with obesity related type 2-diabetes and that limiting its activity can prevent these pathologies. In our study, HFHS significantly increases body weight and fasting glucose levels, starting and progressing after 8 weeks of diet. We observed increased aortic and plasma arginase activity, increased plasma lipid peroxidase activity (measure of systemic oxidative stress), impaired endothelial function (relaxation response to acetylcholine in isolated aortic rings) and arterial stiffness (aortic pulse wave velocity) in HFHS diet mice but not in HFHS mice treated with ABH. Aortic perivascular collagen deposition was significantly higher in HFHS mice compared to normal diet and HFHS+ABH mice. Furthermore, marked increase in circulating blood monocyte (flow cytometry) and macrophage (CD68+) infiltration into the aortic walls was observed. HFHS mice treated with ABH did not show these effects. In conclusion, elevated arginase activity is involved with the pathophysiology of obesity induced type 2 diabetes. Treatment with arginase inhibitor ABH ameliorates HFHS induced endothelial dysfunction, arterial stiffening, oxidative stress, and aortic macrophage infiltration.