An ink composition in which particles of a functional substance comprising a dye or a pigment included in polymer micelles comprising a block polymer are dispersed in a solvent, wherein the particles have an average particle size of not more than 200 nm and a particle size distribution (index of dispersion degree .mu./G.sup.2) of not more than 0.1, a method of forming an image and an image forming apparatus using the same are provided. The main component of the polymer micelle is an amphiphilic block polymer.