The class A amphipathic a-helical peptide 18A is known to form discoidal phospholipid complexes (nanodiscs) similar to that formed by apolipoprotein A-I (apoA-I). To reveal the structural differences in nanodiscs formed with this protein and peptide, we prepared nanodiscs with 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine and applied fluorescence techniques to these nanoparticles. Fluorescence resonance energy transfer between 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine-N-(7-nitro-2-1,3-benzoxadi azol-4-yl) and 1,2-dioleoyl-snglycero-3-phosphoethanolamine-N-(lissamine rhodamine B sulfonyl) in nanodiscs revealed that lipid exchange with 18A nanodiscs is mediated by collisions between nanodiscs. The fluorescence lifetime of dansyl phosphatidylethanolamine and excimer fluorescence of 1,2-bis(1-pyrenedecanoyl)-sn-glycero-3-phosphocholine showed that the degree of hydration of the membrane surface and lateral pressure of acyl chains in 18A nanodiscs are independent of the disc size, suggesting that 18A nanodiscs form planar lipid bilayers irrespective of their size, which differs from apoA-I nanodiscs, whose bilayer deforms to a saddle surface with decreasing size. These results suggest that the flexible structure of a chain of helices in apoA-I is crucial for the formation of saddle surfaces in nanodiscs.