In order to improve the utilization rate and nutritional value of oat bran， with oat bran coarse powder as raw material， and the effects of micronization on the nutritional， functional and antioxidant properties of oat bran were explored. The results showed that with the decrease of bran size， the fat content of oat bran increased significantly （P < 0.05）， the polysaccharide content increased first and then decreased， the polyphenol content increased first and then decreased， and there were significant differences among the groups， and the dietary fiber content decreased significantly （P < 0.05）. With the decrease of the grain size of oat bran， the total antioxidant capacity of oat bran increased gradually， while the scavenging rate of hydroxyl radical and TBA value decreased gradually. The scavenging rate of DPPH radical and ABTS radical reached the maximum when oat bran was 74 μm （200 mesh）， which was 92.1% and 71.88%， respectively. The inhibition rate of linoleic acid peroxidation was the strongest. Correlation analysis showed that there was a great correlation between total phenolics， dietary fiber and antioxidant indexes， and the correlation coefficients were greater than 0.71. Polysaccharides， total phenolics and dietary fiber may be important functional components of oat bran with antioxidant capacity.