The structural change of chicken egg white lysozyme in aqueous 1-butyl-3-methylimidazolium nitrate ([bmim][NO3]) solutions (0-24 M) has been investigated by optical spectroscopy and small-angle X-ray scattering (SAXS) methods. Fourier-transform infrared (FTIR) and circular dichroism (CD) spectra and SAXS profiles indicated that the addition of up to 6 M of [bmim][NO3] induces unfolding of lysozyme resulting from disruption of the alpha-helix by the NO3- ion. On the other hand, even with the addition of more than 10 M of [bmim][NO3], lysozyme aggregation is inhibited and the protein adopts a partially globular state (the secondary structure is partially refolded while the tertiary structure is disrupted). Observation of the structural features of the aqueous [bmim][NO3] solution by Raman OD stretching spectra indicated that bulk-like water still remains at concentrations above 10 M and form an "aggregated water" (water pool) in the nanoheterogeneous structure consisting of a polar domain (the high charge-density region) and nonpolar areas (the alkyl-chain region) in the IL. At these concentrations (above 10 M), lysozyme is not sufficiently hydrated because of the reduced number of water molecules. Consequently lysozyme above 10 M assumes the partially globular state. We propose that the changes of the unique IL solution structure (nanoheterogeneous) between the lower and higher [bmim][NO3] concentrations strongly correlated to the differences in the protein stability of the present results.