Abstract: According to the different macroscopic characteristics of deseeded naked oat straw, it was divided into six parts: internode, joint, cob, leaf, leaf sheath and lemma. The tissue structure, cell morphology and fiber quality of each part were analyzed using light microscope and fiber analyser. The chemical composition of each part was determined, and the distribution of lignin in each part was observed by staining method and fluorescence microscope. The results of the study showed that the internodes were the main physiological parts of naked oat straw, with a mass ratio of 57.4%. There was a high degree of heterogeneity in the biological structure and chemical composition of the various parts. The lignification degree of each tissue showed a rule of vascular tissue>mechanical tissue>thin-walled tissue, and the lignification degrees of internodes, rachis and whole nodes were higher than those of other three fractions. The internode had low benzene-alcohol extractive and ash content, high lignin content of 22.5%, simplest cellular composition, high fiber content, good fiber quality, and most potential for biorefining as a lignocellulosic feedstock.