The present invention relates to a nano-composite containing anionic and cationic polymers and a method for preparing thereof, and more particularly to a poly-gamma-glutamic acid (γ-PGA)/chitosan/manganese iron oxide nanoparticle composite prepared by encapsulating iron oxide-based nanoparticles in a γ-PGA/chitosan polymer composite using the ionic self-assembly properties of poly-gamma-glutamic acid (γ-PGA) and chitosan, which are biocompatible polymer materials, and to a method for preparing thereof.The present invention provides a magnetic resonance imaging nano-contrast agent based on a nanoparticle composite including iron oxide-based nanoparticles encapsulated in a self-assembled composite of anionic poly-gamma-glutamic acid (γPGA) and cationic chitosan. The iron oxide-based nanoparticles encapsulated in the composite interact with each other to produce a synergistic effect on the amplification of magnetic resonance signals, and thus have an relatively excellent contrast effect compared to when they exist as single particles.