Abstract: To investigate the effect of particle size on the structure of oat bran and to study the adhesion preference of a common probiotic Bifidobacterium animalis A6 to oat bran with different particle sizes, this study exhaustively characterized the composition, structure, powder properties and pore characteristics of oat bran with different particle sizes (0.8-1.5 mm, 384.02 μm, 95.51 μm and 41.32 μm), assessed the ζ-potential and rheological properties of oat bran digesta after in vitro digestion, characterized the Bifidobacterium animalis A6 adhesion on oat bran digesta using scanning electron microscopy, and finally explored its potential to adhere to probiotic bacteria by co-precipitation method. The results showed that the protein, starch, and fat content of the four groups of oat bran samples did not change significantly, but the total dietary fiber content decreased with decreasing particle size as the particle size decreased. The fiber matrix of oat bran was disrupted and insoluble dietary fiber continuously converted to soluble dietary fiber. The flow ability decreased, and the MOB had the highest bulk density and vibrational density as well as the jet flow index. Grinding effectively changed the pore structure of the bran and increased the porosity of the bran, where UOB produced microporosity with the diameter less than 10 μm. The absolute value of ζ-potential of oat bran meal decreased with decreasing particle size. Oat bran with median particle size exhibited the highest apparent viscosity, while the ultrafine oat bran was closest to the aqueous solution. Bifidobacterium animalis A6 showed the highest adhesion ability on flaked bran (0.8-1.5 mm) and medium-size bran. In conclusion, oat bran with different particle sizes had differential structural and physical properties, exhibited different rheological properties after in vitro digestion and large particle sizes of oat bran can specifically improve the adhesion of Bifidobacterium animalis A6.