The recently developed photoinitiated atom transfer radical polymerization has been implemented in the inverse microemulsion polymerization of oligo(ethylene glycol) monomethyl ether methacrylate. Two different mechanisms including a simultaneous reverse and normal initiated ATRP and a combination of activators generated by electron transfer and initiators for continuous activator regeneration ATRP are investigated under UV light. The effects of various reaction parameters on the resulting particles have been systematically investigated, and it has been demonstrated that the particle size can be tuned based on UV light, initial stoichiometry, and aqueous phase fraction. The formed particles contained well-defined water-soluble polymers with relatively narrow molecular weight distribution (Mw/Mn < 1.4).