An azo polymerization initiator composition obtained by kneading an azo compound, a dispersing agent and a surface active agent at a temperature of 20.degree. C. or lower, while adjusting the water content of the composition to 0 to 50% by weight, has good storage stability for a long period of time and, at any time desired for polymerization, can give a uniform aqueous suspension of the azo compound for polymerization of unsaturated vinyl monomers with high polymerization initiator activity.