This invention provides a process for producing an aqueous resin dispersion in which the average particle diameter of the dispersed resin is not more than 500 nm, comprising finely dispersing a monomeric mixture (I) containing (A) fatty acid-modified polymerizable unsaturated monomer and (B) other polymerizable unsaturated monomer which is copolymerizable with the monomer (A) in an aqueous medium in such a manner that the dispersed particles have an average particle diameter not more than 500 nm, and polymerizing the resulting emulsified product. The invention also provides aqueous resin compositions which comprise the aqueous resin dispersions, and water-based paint compositions which comprise the aqueous resin compositions.