Water soluble polypeptide, poly[N 5-(2-hydroxyethyl) L-glutamine] (PHEG), was hydrophobiocally modified partially along the main chain by long alkyl groups -(CH2) n–1CH3 (Cn) as side chains. Association and viscoelastic behavior of solutions of these self-assembling polymers (PHEG-g-Cn, n = 12, 16 and 18) were investigated by means of steady-flow viscosity and linear dynamic viscoelasticity measurements. In the mixed solvent of water/ethylene glycol (EG), the main chain of PHEG-g-Cn changed its conformation from flexible random coil to rodlike α-helix with the increase in EG content of the solvent. When the solvent was pure water, existence of the associative alkyl chains induced a drastic increase in shear flow viscosity (η) than PHEG homopolymer, probably because of formation of self-assembled large aggregates via intermolecular association. When EG was used as solvent, η and the elastic storage modulus (G′) of the solution revealed a unique concentration dependence, i.e., η and G′ of PHEG-g-C18 solution at 20 wt% were smaller than those at 15 wt%. These viscoelastic behaviors may be described by the α-helical rodlike conformation of PHEG main chain, which is suitable to form an ordered anisotropic phase like lyotropic liquid crystal, with destruction of a physically crosslinked network structure.