Physical and structural characteristics of milled oat, extruded oat and a mixture of milled oat with bran were investigated mainly by small angle X-ray scattering method (SAXS). This method allowed to examine the lamellar structure of native milled oat as well as other parameters, which can describe the structure of oat extrudates. Decomposition of starch lamellar structure during the extrusion process and formation of many inhomogeneities (heterogeneities) in extrudates were observed. The existence of fractal structure for these inhomogeneities was also confirmed by the SAXS method. It was also observed that, there were structural differences between extrudates obtained from milled oat and from a mixture of milled oat and its bran.