In the current study, we evaluated total phenolics, phenolic profile, and oxygen radical absorbance capacity (ORAC) of onion (Allium cepa L.). The inhibitory activity of onion extracts against porcine pancreatic ${alpha}$-amylase and rat intestinal ${alpha}$-glucosidase was also investigated. Ethyl alcohol extract of onion skin had the highest ${alpha}$-glucosidase inhibitory activity, ORAC value and total phenolic content, followed by water extract of skin, ethyl alcohol extract of pulp, and water extract of pulp. The ${alpha}$-glucosidase inhibitory activity of the extracts was compared to selected specific phenolics detected in the extracts using high performance liquid chromatography (HPLC). Quercetin, a major phenolic compound in onion extract had high ${alpha}$-glucosidase inhibitiory activity. The ${alpha}$-glucosidase inhibitory activity of the onion extracts correlated to the phenolic content and antioxidant activity of the extracts. These results suggest that onion which has high quercetin content has the potential to contribute as a dietary supplement for controlling hyperglycemia and oxidative stress-linked diabetes complications.