The amino acid compositions, secondary structure, and self-assembly of oat protein isolate (OPI), which was purified from the high-protein Chinese oat, have been investigated by using a combination of amino acid analysis, Fourier transform infrared spectroscopy (FTIR), and tapping mode atomic force microscopy (TP-AFM). OPI, with molecular weights ranging from 14.0 kDa to 66.0 kDa, was rich in essential amino acids and contained 24.7% glutamic acid and 8.1% leucine. The amino acid contents of OPI are 4.5-8.7 times higher than those of oat flour. The secondary structures of OPI have been quantified by the deconvolution of the amide I band of the FTIR spectrum of OPI, which were found to contain approximately 7% beta-turn, 19% alpha-helix, and 74% beta-sheet. Tapping mode AFM results further suggest that the oat protein isolate has two major types of shapes, ellipsoidal and disk-like. At protein concentrations below 0.5 mg/mL, most of the OPI molecules are in the isolated form. However, when the concentration of OPI reaches 1.0 mg/mL, some of the OPI molecules self-assembled into large and heterogeneous protein aggregates.