Dextran can self-assemble in water when modified with acetylated groups. Self-assembly of acetylated dextran (Ac-DEXs) is monitored by light scattering method. Molecular weight and radius of gyration of dextran increase with acetylation degree. The aggregates of Ac-DEXs show hard spherical nanoparticles. The highest acetylated substitution represents the formation of gel nanoparticles.