To provide an amphipathic polymer particle which functions as both a dispersant and a binder in a water-based ink composition and to provide a process for preparation thereof.

The present invention relates to an amphipathic polymer particle which functions as both a dispersant and a binder in a water-based ink composition, to an ink composition containing the above polymer particle, and to a process for preparing the above particle. The polymer particle contains both of the hydrophilic part and the hydrophobic part which have previously determined structures, and has an average particle diameter of 50-500 nm. The amphipathic polymer particle can be prepared by the side chain interconversion method containing either a crosslinker or no crosslinker, or by a polymerization process accompanied with the ATRP step. The present invention improves the respective stability of the polymer and ink owing to both of the ionic stability and steric one of the suspended polymer particle.