Several well-defined graft copolymers (7) having a water-soluble natural polysaccharide (dextran) stock and hydrophilic polyamide branches of different molecular weights were synthesized by using a quasi-living anionic polymerization of a bicyclic lactam 1, namely by the reaction of the acyllactam group in the quasi-living polyamide 2 with the hydroxyl and amino groups in the dextran derivative (6), of which the reducing end was converted to an amino group. An oligosaccharide-terminated polyamide 3 was also prepared efficiently by similar coupling reaction of the corresponding quasi-living polyamide 2 with the maltose derivative (5) having an amino group.