This letter reports a novel, facile method of fabricating hollow polymer microspheres based on the phase-inversion method. In this approach, when hydrophobic chlorinated polypropylene was grafted with methyl methacrylate, butyl acrylate, and acrylic acid via free-radical polymerization and then neutralized by triethylamine and gradually diluted with deionized water, phase inversion happened, directly yielding hollow polymer microspheres. SEM, TEM, and optical images confirmed the hollow structure. A formation mechanism of the hollow polymer microspheres was proposed.