To evaluate the effects of naked oat cultivars on their phenolic contents and antioxidant activities, the oat seeds from 21 different cultivars in China were collected, and their major nutritional components, compositions of polyphenols, and antioxidant activities were measured. As the results, oat cultivars affected its phenolic composition and antioxidant activity significantly. Vanillin, chlorogenic acid, ferulic acid, avenanthramide C (Bc), avenanthramide A (Bp), and avenanthramide B (Bf) in oat polyphenols extracts were detected in all cultivars of oat, while caffeic acid, p-coumaric acid and rutin were partly detected. All oat cultivars showed significant free radical scavenging activity, although their activities were lower than that of vitamin C (VC). Different oat varieties showed different antioxidant activities, among which Yanke 1 had the best ability to scavenge •OH ((58.78±1.99) μg mL-1), Baiyan 10 showed the strongest DPPH• ((22.00±0.43) μg mL-1) and ABTS•+ ((6.92±0.44) μg mL-1) scavenging activity, and Dingyou 1 showed good scavenging ability of DPPH• ((22.71±0.42) μg mL-1) and •OH ((81.50±1.73) μg mL-1). In addition, the correlation coefficients of contents between the major nutritional components in different cultivars of oat and phenolic compounds indicated that determination of main nutrients could be used as an easy technique to evaluate approximately the contents and compositions of oat polyphenols, which simplifies the selection of good oat cultivar.