Stabilization of the alpha-helix structure of the poly(L-lysine) segment in a poly(ethylene glycol)-poly(L-lysine) block copolymer was evidenced in aqueous medium through circular dichroism (CD), H-1-NMR, and static light scattering (SLS) measurements. It was revealed that poly(L-lysine) oligomers (M(w) = 2450), which themselves cannot form an alpha-helix structure due to substantially lower molecular weight, form an alpha-helix structure under high-pH condition when they are conjugated with poly(ethylene glycol). Moreover, it has been suggested by static light scattering that the block copolymer under high-pH condition exists as a dimer with alpha-helical poly(L-lysine) segments. The alpha-helix content in the block copolymer gradually decreased in a linear manner with the addition of urea, which is in sharp contrast to the high molecular weight poly(L-lysine) (M(w) = 170 000), showing a steep transition from an alpha-helix structure to a random coil at a critical urea concentration (1.0 M). To completely shift the conformation of the block copolymer to a random coil, 3.0 M urea is needed, suggesting a substantial significance of the stabilization effect. From BD-NOESY H-1-NMR and static light scattering measurement, it was suggested that the block copolymers selectively form dimers with a micelle-like structure : a poly(ethylene glycol) shell segregates the poly(L-lysine) segment from the aqueous medium to stabilize the alpha-helix structure.