Provided are a mussel adhesive protein-mimetic dispersion stabilizing agent to disperse nanoparticles in an aqueous medium, a colloidal solution including nanoparticles dispersed and stabilized by the dispersion stabilizing agent, and a contrast agent including the colloidal solution. More particularly, the mussel adhesive protein-mimetic dispersion stabilizer is a polyethyleneimine-graft-(poly-ethyleneglycol；polyDOPA) PEI-graft-(PEG；PDOPA). The graft polymer is formed of two parts. One is polyethyleneglycol grafted with a polyethyleneime which has an affinity to an aqueous medium, and the other is polyDOPA which has an affinity to the surface of nanoparticles. Because of those characteristics, the stabilizer shows a stable dispersion of nano particles in the aqueous medium.
